To assess the efficacy and safety of Alefacept compared to placebo for the treatment of Chronic Plaque Psoriasis in patients for whom conventional therapies are ineffective and inappropriate

Patients who completed the initial 24-week treatment course (12 weeks of weekly dosing following b y 12 weeks of follow-up) and met the eligibility criteria continued in the 24-week open label extension

Inclusion Criteria:

• Diagnosis of Chronic Plaque Psoriasis involving ≥ 10% body surface area
• Treatment history for Psoriasis in which 3 or more conventional anti-psoriatic therapies are inappropriate or ineffective
• CD4+ T lymphocyte counts at or above the lower limit of normal

Exclusion Criteria:

• Clinically significant abnormal hematology values or blood chemistry values
• AST or ALT ≥ 3x the upper limit of normal
• Other types of Psoriasis
• Serious infection within the 3 months prior to the first dose of study drug
• History of drug or alcohol abuse within the past 2 years
• Antibody positive for HIV
• History of malignancy
• History of any clinically significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal, and/or other major disease
• Exposure to psoriasis-modifying phototherapy or high-potency topical corticosteroid therapy within 2 weeks prior to the first dose of study drug
• Exposure to systemic anti-psoriatic therapy , topical immunomodulators or topical CNIs within 4 weeks prior to the first dose of study drug
• Current treatment with any therapy for tuberculosis
• Previous exposure to Alefacept
• Nursing mothers, pregnant women, and women planning to become pregnant during the study