Alefacept is the first biologic agent approved for the treatment of chronic plaque psoriasis in the United States. Alefacept, administered intravenously (i.v.) or intramuscularly (i.m.), was found to be well tolerated, safe, and efficacious in two pivotal phase 3 studies in patients with moderate to severe psoriasis. Treatment with i.v. alefacept was associated with a median duration of off-treatment response of 216 days (approximately 7 months). In a follow-up extension study to the phase 3 i.m. study, duration of therapeutic response was also examined. Patients who achieved a > or = 75% reduction in baseline Psoriasis Area and Severity Index (PASI 75) with the first course of alefacept 15 mg i.m. in the phase 3 study maintained a PASI 50 for a median duration of 209 days. In addition, the extension study demonstrated that a second course of i.m. alefacept is safe and well tolerated in patients with psoriasis.