Agents that safely provide long-term control of psoriasis are needed. To determine the safety and efficacy of extended efalizumab therapy, 339 patients with moderate to severe chronic plaque psoriasis received 2 mg/kg subcutaneous (SC) efalizumab weekly for 12 weeks. At Week 12, 290 patients who achieved > or =50% Psoriasis Area and Severity Index (PASI-50) improvement or a static Physician's Global Assessment grading of "mild," "minimal," or "clear" entered maintenance treatment with weekly SC efalizumab. At Week 12, 82%, 41%, and 13% of 339 patients achieved a PASI-50, PASI-75, and PASI-90 response, respectively. At 15 months, 65%, 50%, and 25% of patients achieved a PASI-50, PASI-75, and PASI-90 response, respectively (intent-to-treat, n = 339). The incidence of adverse events did not increase over time, and no new common adverse events were reported. The majority of patients experienced sustained efficacy with no increase in toxicity. This study is planned to continue; patients will receive up to 36 months of continuous efalizumab.