Context Because T-cell interactions are involved in the pathophysiology of psoriasis, therapy with a T-cell modulator may have beneficial effects on psoriasis severity and health-related quality of life (HRQL). Objective To assess the efficacy and safety of efalizumab, a T-cell modulator, in patients with plaque psoriasis. Design, Setting, and Patients Phase 3 randomized, double-blind, parallelgroup, placebo-controlled trial involving 556 adult patients with stable, moderate to severe plaque psoriasis and conducted at 30 study centers in the United States and Canada between January and July 2002. Interventions Patients were randomly assigned in a 2:1 ratio to receive 12 weekly doses of subcutaneous efalizumab, 1 mg/kg (n=369), or placebo equivalent (n=187). Main Outcome Measures At least 75% improvement on the Psoriasis Area and Severity Index (PASI-75); improvement on the overall Dermatology Life Quality Index (DLQI), Itching Visual Analog Scale (VAS), and Psoriasis Symptom Assessment (PSA) at week 12 vs baseline. Results Efalizumab-treated patients experienced significantly greater improvement on all end points than placebo-treated patients. Twenty-seven percent of efalizumabtreated patients achieved PASI-75 vs 4% of the placebo group (PϽ.001). Efalizumabtreated patients exhibited significantly greater mean percentage improvement than placebo-treated patients on the overall DLQI (47% vs 14%; PϽ.001), Itching VAS (38% vs −0.2%; PϽ.001), and PSA frequency and severity subscales (48% vs 18% and 47% vs 17%, respectively; PϽ.001 for both) at the first assessment point. Efalizumab was safe and well tolerated, with primarily mild to moderate adverse events. Conclusion In this 12-week study, efalizumab resulted in significant improvements in clinical end points, including physician-assessed and dermatology-specific patientreported HRQL measures, in patients with moderate to severe plaque psoriasis.