Tumour necrosis factor alpha (TNFalpha) is thought to play a part in the pathogenesis of psoriasis. We assessed the efficacy and safety of continuous treatment with infliximab, a monoclonal antibody that binds to and neutralises the activity of TNFalpha, in patients with psoriasis. In this phase III, multicentre, double-blind trial, 378 patients with moderate-to-severe plaque psoriasis were allocated in a 4:1 ratio to receive infusions of either infliximab 5 mg/kg or placebo at weeks 0, 2, and 6, then every 8 weeks to week 46. At week 24, placebo-treated patients crossed over to infliximab treatment. Skin and nail signs of psoriasis were assessed using the psoriasis area and severity index (PASI) and nail psoriasis severity index (NAPSI), respectively. The primary endpoint, analysed on an intention-to-treat-basis, was the proportion of patients achieving at least a 75% improvement in PASI from baseline to week 10. At week 10, 80% (242/301) of patients treated with infliximab achieved at least a 75% improvement from their baseline PASI (PASI 75) and 57% (172/301) achieved at least a 90% improvement (PASI 90), compared with 3% and 1% in the placebo group, respectively (p<0.0001). At week 24, PASI 75 (82% for infliximab vs 4% for placebo) and PASI 90 (58%vs 1%) were maintained (p<0.0001). At week 50, 61% achieved PASI 75 and 45% achieved PASI 90 in the infliximab group. Infliximab was generally well tolerated in most patients. Infliximab is effective in both an induction and maintenance regimen for the treatment of moderate-to-severe psoriasis, with a high percentage of patients achieving sustained PASI 75 and PASI 90 improvement through 1 year.