Safety and Tolerability of Repeat Courses of IM Alefacept

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.