The purpose of this study is to determine whether repeat courses of alefacept, administered intramuscularly, are safe when given to chronic plaque psoriasis patients who are receiving standard dermatology treatments.

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.