Inclusion Criteria:

- Age ≥18

- histologically confirmed adenocarcinoma of the prostate.

- Orchidectomy within the four previous weeks to enter the study.

- bone metastasis evidenced by bone scan.

- A hip DEXA study with a DS < 3.

- No hormonal therapy previous to enter the study.

- ECOG performance score of 0-2.

- Signed written informed consent.

Exclusion Criteria:

- Abnormal renal function evidenced by a creatinine clearance ≤60 ml/min.

- Any kind of hormonal therapy for prostate cancer previous to enter the study.

- Serum calcium corrected for albumin level < 8.0 mg/dl.

- WBC < 3.0x10^3, ANC < 1500/mm3, Hemoglobin < 8.0 g/dl, platelets < 75 x 10^3/l.

- Abnormal hepatic function evidenced by ALT and AST value >2.5 UNL

- Subjects with any other malignant disease that can affect the bone.

- Subjects with any other non malignant disease that can jeopardize the evaluation of
the primary objectives of this trial (severe osteoporosis) or that do not aloud
perform the trial evaluations.

- Known hypersensibility to zoledronic acid or other bisphosphonates.

- Subjects that in the investigator's opinion can not cooperate with the protocol.

Other protocol inclusion/exclusion criteria may apply.