BACKGROUND: Tildrakizumab is a high-affinity, humanised, IgG1 κ antibody targeting interleukin 23 p19 that represents an evolving treatment strategy in chronic plaque psoriasis. Previous research suggested clinical improvement with inhibition of interleukin 23 p19. We did two phase 3 trials to investigate whether tildrakizumab is superior to placebo and etanercept in the treatment of chronic plaque psoriasis. METHODS: We did two three-part, parallel group, double-blind, randomised controlled studies, reSURFACE 1 (at 118 sites in Australia, Canada, Japan, the UK, and the USA) and reSURFACE 2 (at 132 sites in Europe, Israel, and the USA). Participants aged 18 years or older with moderate-to-severe chronic plaque psoriasis (body surface area involvement ≥10%, Physician's Global Assessment [PGA] score ≥3, and Psoriasis Area and Severity Index [PASI] score ≥12) were randomised (via interactive voice and web response system) to tildrakizumab 200 mg, tildrakizumab 100 mg, or placebo in reSURFACE 1 (2:2:1), or to tildrakizumab 200 mg, tildrakizumab 100 mg, placebo, or etanercept 50 mg (2:2:1:2). Randomisation was done by region and stratified for bodyweight (≤90 kg or >90 kg) and previous exposure to biologics therapy for psoriasis. Investigators, participants, and study personnel were blinded to group allocation and remained blinded until completion of the studies. Assigned medication was identical in appearance and packaging. Tildrakizumab was administered subcutaneously at weeks 0 and 4 during part 1 and at week 16 during part 2 (weeks 12 and 16 for participants re-randomised from placebo to tildrakizumab; etanercept was given twice weekly in part 1 of reSURFACE 2 and once weekly during part 2). The co-primary endpoints were the proportion of patients achieving PASI 75 and PGA response (score of 0 or 1 with ≥2 grade score reduction from baseline) at week 12. Safety was assessed in the all-participants-as-treated population, and efficacy in the full-analysis set. These trials are registered with ClinicalTrials.gov, numbers NCT01722331 (reSURFACE 1) and NCT01729754 (reSURFACE 2). These studies are completed, but extension studies are ongoing. FINDINGS: reSURFACE 1 ran from Dec 10, 2012, to Oct 28, 2015. reSURFACE 2 ran from Feb 12, 2013, to Sept 28, 2015. In reSURFACE 1, 772 patients were randomly assigned, 308 to tildrakizumab 200 mg, 309 to tildrakizumab 100 mg, and 155 to placebo. At week 12, 192 patients (62%) in the 200 mg group and 197 patients (64%) in the 100 mg group achieved PASI 75, compared with 9 patients (6%) in the placebo group (p<0·0001 for comparisons of both tildrakizumab groups vs placebo). 182 patients (59%) in the 200 mg group and 179 patients (58%) in the 100 mg group achieved PGA responses, compared with 11 patients (7%) in the placebo group (p<0·0001 for comparisons of both tildrakizumab groups vs placebo). In reSURFACE 2, 1090 patients were randomly assigned, 314 to tildrakizumab 200 mg, 307 to tildrakizumab 100 mg, 156 to placebo, and 313 to etanercept. At week 12, 206 patients (66%) in the 200 mg group, and 188 patients (61%) in the 100 mg group achieved PASI 75, compared with 9 patients (6%) in the placebo group and 151 patients (48%) in the etanercept group (p<0·0001 for comparisons of both tildrakizumab groups vs placebo; p<0·0001 for 200 mg vs etanercept and p=0·0010 for 100 mg vs etanercept). 186 patients (59%) in the 200 mg group, and 168 patients (59%) [corrected] in the 100 mg group achieved a PGA response, compared with 7 patients (4%) in the placebo group and 149 patients (48%) in the etanercept group (p<0·0001 for comparisons of both tildrakizumab groups vs placebo; p=0·0031 for 200 mg vs etanercept and p=0·0663 for 100 mg vs etanercept). Serious adverse events were similar and low in all groups in both trials. One patient died in reSURFACE 2, in the tildrakizumab 100 mg group; the patient had alcoholic cardiomyopathy and steatohepatitis, and adjudication was unable to determine the cause of death. INTERPRETATION: In two phase 3 trials, tildrakizumab 200 mg and 100 mg were efficacious compared with placebo and etanercept and were well tolerated in the treatment of patients with moderate-to-severe chronic plaque psoriasis. FUNDING: Merck & Co.