Inclusion Criteria:

- Written informed consent.

- At least 16 years of age.

- Diagnosed with chronic plaque psoriasis and require systemic therapy.

- CD4+ lymphocyte counts at or above the lower limit of normal for Covance Central
Laboratory Services, Inc., unless on a stable dose of prednisone (counts must be at
or above 300 cells/mm3).

Exclusion Criteria:

- Unstable erthrodermic or pustular psoriasis.

- Diagnosis of guttate psoriasis.

- Serious local infection or systemic infection within 3 months prior to first dose of
alefacept.

- Positive for HIV antibody.

- Known invasive malignancy within 5 years of enrollment. Patients with a history of
treated squamous cell and/or basal cell carcinomas limited to the skin are not
excluded.

- Evidence of active tuberculosis.

- Current treatment for active tuberculosis or tuberculosis prophylaxis.

- Female patients unwilling to practice effective contraception as defined by the
investigator.

- Female patients who are pregnant or breast-feeding.

- Current enrollment in any other investigational drug study.

- Previous participation in this study or previous alefacept studies.