Inclusion Criteria:

- Adult men at least 18 years of age at the time of randomization.

- Metastatic, histologically or cytologically confirmed adenocarcinoma of the prostate
that has progressed after start of androgen deprivation therapy, which includes prior
orchiectomy or medical castration using leuteinizing hormone-releasing hormone (LHRH)
antagonists such as leuprolide or goserelin (patients must remain on LHRH analogue
therapy for the duration of the study if not surgically castrated). Progressive
disease should be documented by:

a. PSA progression (defined as two consecutive increases in PSA over a previous
reference value, with the first increase in PSA occurring at a minimum of 1 week
after the reference value [obtained within 2 months prior to study randomization] and
confirmed by a subsequent increase in PSA whose value must be ³ 5 ng/mL prior to
study randomization);41 and one of the following: i. Bone metastases (defined as ³3
foci on bone scan and confirmed radiologically within 1 month prior to study
randomization); or ii. Measurable non-bony metastatic disease (documented by
radiographic studies performed within 1 month prior to study randomization).

- Serum testosterone levels <50 ng/dL documented in non-surgically castrated patients
within 21 days prior to randomization.

- Prior treatment with nonsteroidal antiandrogens (e.g., flutamide or bicalutamide) is
allowed provided:

- There is evidence of disease progression (defined in Inclusion Criteria #2) following
withdrawal of antiandrogens; and b. At least 4 weeks for flutamide or 6 weeks for
bicalutamide have passed since last treatment.

- Prior treatment with ketoconazole and/or steroids is allowed provided at least 4
weeks have passed since last treatment. There are no restrictions for use of
prednisone (5 mg twice daily) or another functionally equivalent oral corticosteroid
for treatment of pain.

- In the rare instance a patient is potent, he must agree to practice an effective
method of contraception including condom or abstinence, unless his sexual partner is
sterile, from the time of first administration of MEDI-522 or docetaxel through 30
days after the last dose of either docetaxel or MEDI-522, whichever is the last drug
discontinued.

- Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 documented
within 21 days prior to randomization.

- Life expectancy, in the opinion of the investigator, of at least 6 months.

- White blood cell (WBC) count ≥ 3,000/mm3; absolute neutrophil count (ANC) ≥
1,500/mm3; platelet count ≥ 100,000/mm3; and hemoglobin ³ 9 g/dL documented within 21
days prior to randomization.

- Bilirubin ≤ ULN; aspartate transaminase (AST)/alanine transaminase (ALT) £1.5 times
ULN or if AST/ALT is >1.5 times ULN, then alkaline phosphatase must be £2.5 times
ULN; serum creatinine ≤ 1.5 mg/dL; INR within normal range, unless a patient is
receiving anticoagulation therapy; and corrected serum calcium between 8.0-11.5 mg/dL
documented within 21 days prior to randomization.

- Patients who had prior major surgery are eligible if at least 4 weeks have passed
since their surgery and all surgical wounds have healed prior to study randomization.

- Prior radiotherapy including therapeutic isotopes is allowed provided measurable or
evaluable disease that is clearly progressing is present and all acute
radiation-related toxicities have resolved prior to study randomization.

- Prior treatment with unconventional therapy for malignancy (e.g., vitamins, St.
John's Wort, PC-SPES, saw palmetto, or other herbal remedies) is allowed provided at
least 4 weeks have passed since last treatment prior to randomization.

- Written informed consent and HIPAA authorization (USA sites only) obtained from the
patient prior to receipt of any study medication or beginning study procedures.

Exclusion Criteria:

- Prior chemotherapy for metastatic prostate cancer (prior adjuvant chemotherapy is
allowed provided it is non-taxane based and at least 6 months have passed since last
treatment).

- Prior treatment with other investigational agents within 4 weeks prior to
randomization.

- Planned concurrent treatment with unconventional therapy for malignancy (e.g.,
vitamins, St. John's Wort, PC-SPES, saw palmetto, or other herbal other herbal
remedies) based on medical history. Currently requiring anticoagulation (excluding
use of heparin flush solutions for maintenance of catheter lines) for any
thromboembolic disease based on medical history and physical examination.

- Current or planned participation (from the time of randomization through 30 days
after the last dose of either docetaxel or MEDI-522, whichever is the last drug
discontinued) in a research protocol in which an investigational agent or therapy may
be administered.

- Any evidence of or history elicited by the investigator of prior treatment with
MEDI-522 or MEDI-523.

- Prior treatment with calcitonin, mithramycin, or gallium nitrate within 2 weeks prior
to randomization.

- Clinically evident central nervous system (CNS) metastasis.

- History of prior malignancies within the past 5 years other than adequately treated
basal cell or squamous cell skin cancer or Stage I or II cancer currently in complete
remission;

- Any evidence of or history elicited by the investigator of symptomatic
cerebrovascular events (i.e., stroke or transient ischemic attack) within 6 months
prior to randomization; or any history or evidence of pulmonary embolism or
thrombophlebitis (including deep vein thrombosis) requiring anticoagulant therapy
(e.g., warfarin or heparin).

- Any evidence of or history elicited by the investigator of myocardial infarction or
angina within 6 months prior to randomization.

- Any evidence of or history elicited by the investigator of hematemesis, melena,
hematochezia, or uncontrolled gross hematuria within 4 weeks prior to randomization.

- Any evidence of or history elicited by the investigator of bleeding diatheses.

- Major elective surgery planned from the time of randomization through 30 days after
the last dose of either docetaxel or MEDI-522, whichever is the last drug
discontinued.

- Any evidence of or history elicited by the investigator of hypersensitivity to a
previously administered monoclonal antibody.

- Any evidence of or history elicited by the investigator of hypersensitivity to drugs
formulated with polysorbate 80, prednisone (or other functionally equivalent oral
corticosteroid), or zoledronic acid.

- Known human immunodeficiency virus (HIV) or known active viral hepatic infections
based on medical history and physical examination.

- Any evidence of or history elicited by the investigator of uncontrolled or refractory
hypertension or uncontrolled diabetes despite medication within 6 months prior to
randomization.

- Any evidence of or history elicited by the investigator of an active infection
requiring parenteral anti-infective therapy.

- A general medical or psychological condition or behavior, including substance
dependence or abuse that, in the opinion of the investigator, might not permit the
patient to complete the study or sign the informed consent.