The efficacy and safety of efalizumab in the treatment of moderate-to-severe chronic psoriasis has been established in studies of up to 3 years' duration. This study aims to describe the efficacy of up to 15 months' treatment with efalizumab and the convenience of therapy in patients with moderate-to-severe chronic plaque psoriasis. Patients who had completed a 3-month, double-blind, randomized, placebo-controlled, Phase IIIb trial entered a 12-month extension study and received efalizumab, 1 mg/kg/week administered subcutaneously, for up to 12 months. Of 450 patients originally randomly assigned to receive efalizumab, 40.9% achieved a reduction of > or = 75% in the Psoriasis Area and Severity Index score after 15 months of treatment. Improvements were also observed on the frequency and severity subscales of the Psoriasis Symptom Assessment. The majority of patients reported that efalizumab treatment was more or much more convenient than other psoriasis treatments. Efalizumab, 1 mg/kg/week, provides long-term efficacy and good convenience with up to 15 months of continuous treatment.