Interleukins 12 and 23 have important roles in the pathophysiology of psoriasis. We assessed ustekinumab, a human monoclonal antibody directed against these cytokines, for the treatment of psoriasis. In this phase III, parallel, double-blind, placebo-controlled study, 766 patients with moderate-to-severe psoriasis were randomly assigned to receive ustekinumab 45 mg (n=255) or 90 mg (n=256) at weeks 0 and 4 and then every 12 weeks; or placebo (n=255) at weeks 0 and 4, with subsequent crossover to ustekinumab at week 12. Patients who were initially randomised to receive ustekinumab at week 0 who achieved long-term response (at least 75% improvement in psoriasis area and severity index [PASI 75] at weeks 28 and 40) were re-randomised at week 40 to maintenance ustekinumab or withdrawal from treatment until loss of response. Both randomisations were done with a minimisation method via a centralised interactive voice response system. The primary endpoint was the proportion of patients achieving PASI 75 at week 12. Analyses were by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00267969. All randomised patients were included in the efficacy analysis. 171 (67.1%) patients receiving ustekinumab 45 mg, 170 (66.4%) receiving ustekinumab 90 mg, and eight (3.1%) receiving placebo achieved PASI 75 at week 12 (difference in response rate vs placebo 63.9%, 95% CI 57.8-70.1, p<0.0001 for 45 mg and 63.3%, 57.1-69.4, p<0.0001 for 90 mg). At week 40, long-term response had been achieved by 150 patients in the 45 mg group and 172 patients in the 90 mg group. Of these, 162 patients were randomly assigned to maintenance ustekinumab and 160 to withdrawal. PASI 75 response was better maintained to at least 1 year in those receiving maintenance ustekinumab than in those withdrawn from treatment at week 40 (p<0.0001 by log-rank test). During the placebo-controlled phase, adverse events occurred in 278 (54.5%) of the 510 patients receiving ustekinumab and 123 (48.2%) of the 255 receiving placebo. Serious adverse events occurred in six (1.2%) of 510 patients receiving ustekinumab and in two (0.8%) of 255 receiving placebo in this phase. The pattern of adverse events was much the same in the placebo crossover and randomised withdrawal phases as it was in the placebo-controlled phase. Ustekinumab seems to be efficacious for the treatment of moderate-to-severe psoriasis; dosing every 12 weeks maintains efficacy for at least a year in most patients.