Background PHOENIX 1 was a phase III, randomized, double-blind, placebo-controlled study that demonstrated the long-term efficacy and safety of ustekinumab in patients with moderate-to-severe psoriasis. Objectives To assess the effect of ustekinumab maintenance therapy on health-related quality of life (HRQoL) in PHOENIX 1 patients. Patients and methods Patients (n = 766) were randomized to receive ustekinumab 45 mg (n = 255) or 90 mg (n = 256) at weeks 0 and 4 and every 12 weeks thereafter, or placebo (n = 255) at weeks 0 and 4 with crossover to ustekinumab at week 12. Ustekinumab-randomized patients achieving at least 75% improvement in Psoriasis Area and Severity Index (PASI) 75 at weeks 28 and 40 were re-randomized at week 40 to continue ustekinumab or be withdrawn until loss of therapeutic effect. HRQoL was assessed using the SF-36 and Dermatology Life Quality Index (DLQI). Results At baseline, more than 97% had a DLQI > 1 and the average DLQI was > 10, indicating a significant impact on patients' HRQoL. Significantly greater proportions of patients receiving ustekinumab 45 and 90 mg achieved a normalized DLQI score (< or = 1) compared with placebo (53.2%, 52.4% and 6.0%, respectively, both P < 0.001) at week 12 and achieved a clinically meaningful improvement (increase of at least five points) in SF-36 physical (23.1%, 33.7% and 15.6%) and mental (25.5%, 31.3% and 14.8%) component summary scores. At week 12, changes in individual DLQI and SF-36 domains were significantly better in each ustekinumab group vs. placebo (P < 0.001). The magnitude of improvement across SF-36 scales was greatest for the bodily pain and social functioning domains. Improvements in HRQoL were sustained with maintenance ustekinumab therapy through at least 1 year. Regression analysis showed that, after adjustment for improvement in PASI or Physician's Global Assessment (PGA), ustekinumab-treated patients demonstrated significant improvements in DLQI. Conclusions Ustekinumab improves HRQoL in patients with moderate-to-severe psoriasis. Patient-reported outcomes measured a treatment effect beyond that indicated by clinical measures.