Alefacept is the first biologic agent approved by the US Food and Drug Administration for the treatment of psoriasis. To date, more than 1000 patients with moderate to severe psoriasis have been enrolled in phase III clinical trials of alefacept. More than 30% of patients treated with 2 courses of alefacept reached a Physician's Global Assessment of clear to almost clear, and approximately 40% and 70% of patients achieved a Psoriasis Area Severity Index score of 75 and 50 after the same regimen. Alefacept is well tolerated, and there have been no reports of significant systemic toxicity or serious treatment-related adverse events.