An extension study to evaluate safety and tolerability of up to 3 additional courses of IM alefacept in patients with chronic plaque psoriasis who have been previously treated with 1 or 2 courses of IM alefacept.

Inclusion Criteria:

• Must have received at least 8 injections in the C99-717 study and completed the final follow-up visit OR,
• Must have completed the C99-712 study and been in C99-717 interim visits at the time dosing in the C99-717 study was closed. A subject who completed C99-712 but did not participate in any part of C99-717, including interim visits, must have prior sponsor approval before admission into C-728

Exclusion Criteria:

• Nursing mothers, pregnant women, and women planning to become pregnant while on study are to be excluded. Female patients who are not postmenopausal for at least 1 year, surgically sterile, or willing to practice effective contraception during the study
• Clinically significant abnormal hematology values or history of an immunosuppressive disorder
• Serious local infection or systemic infection within 3 months prior to the first dose of alefacept
• A significant change in the subject's medical history from their previous alefacept study
• Any subject who initiated alternative systemic therapy, phototherapy, or disallowed therapy prior to visit 8 in study C99-712 or C99-717
• Current enrollment in any investigational study in which the subject is receiving any type of drug, biologic, or non-drug therapy (participation in registry-type studies is allowed)