BACKGROUND: Inhibiting interleukin (IL)-23 in patients with psoriasis has demonstrated high levels of skin clearance. OBJECTIVES: To investigate, in a phase II (AMAF; NCT02899988), multicentre, double-blind trial, the efficacy and safety of three doses of mirikizumab (LY3074828), a p19-directed IL-23 antibody, vs. placebo in patients with moderate-to-severe plaque psoriasis. METHODS: Adult patients were randomized 1 : 1 : 1 : 1 to receive placebo (n = 52), mirikizumab 30 mg (n = 51), mirikizumab 100 mg (n = 51) or mirikizumab 300 mg (n = 51) subcutaneously at weeks 0 and 8. The primary objective was to evaluate the superiority of mirikizumab over placebo in achieving a 90% improvement in the Psoriasis Area and Severity Index (PASI 90) response at week 16. Comparisons were done using logistic regression analysis with treatment, geographical region and previous biological therapy in the model. Missing data were imputed as nonresponses. RESULTS: Ninety-seven per cent of patients completed the first 16 weeks of the study. The primary end point was met for all mirikizumab dose groups vs. placebo, with PASI 90 response rates at week 16 of 0%, 29% (P = 0·009), 59% (P < 0·001) and 67% (P < 0·001) for patients receiving placebo, and mirikizumab 30 mg, 100 mg and 300 mg, respectively. There were two (1%) serious adverse events in mirikizumab-treated patients vs. one (2%) in a placebo-group patient. CONCLUSIONS: At week 16, 67% of patients treated with mirikizumab 300 mg at 8-week intervals achieved PASI 90. The percentage of patients reporting at least one treatment-emergent adverse event was similar among patients treated with placebo or mirikizumab.