Patients with hidradenitis suppurativa have substantial unmet clinical needs and scarce therapeutic options. We aimed to assess the efficacy and safety of bimekizumab, a monoclonal IgG1 antibody that selectively inhibits interleukin (IL)-17F and IL-17A, in patients with moderate-to-severe hidradenitis suppurativa. BE HEARD I and II were two identically designed, 48-week randomised, double-blind, placebo-controlled, multicentre phase 3 trials. Patients aged 18 years or older with moderate-to-severe hidradenitis suppurativa were randomly assigned 2:2:2:1 using interactive response technology (stratified by worst Hurley Stage at baseline and baseline systemic antibiotic use) to receive subcutaneous bimekizumab 320 mg every 2 weeks; bimekizumab 320 mg every 2 weeks to week 16, then every 4 weeks to week 48; bimekizumab 320 mg every 4 weeks to week 48; or placebo to week 16, then bimekizumab 320 mg every 2 weeks. The primary outcome was an hidradenitis suppurativa clinical response of at least 50%, defined as a reduction in total abscess and inflammatory nodule count of at least 50% from baseline with no increase from baseline in abscess or draining tunnel count (HiSCR50) at week 16. Efficacy analyses included all randomly assigned study patients (intention-to-treat population). Safety analyses included all patients who received at least one full or partial dose of study treatment in the safety set, and of bimekizumab in the active-medication set. These trials are registered at ClinicalTrials.gov, NCT04242446 and NCT04242498, and both are completed. Patients for BE HEARD I were recruited from Feb 19, 2020, to Oct 27, 2021, and 505 patients were enrolled and randomly assigned. Patients for BE HEARD II were recruited from March 2, 2020, to July 28, 2021, and 509 patients were enrolled and randomly assigned. The primary outcome at week 16 was met in the group who received bimekizumab every 2 weeks using modified non-responder imputation; higher responder rates were observed with bimekizumab versus placebo in both trials: 138 (48%) of 289 patients versus 21 (29%) of 72 patients in BE HEARD I (odds ratio [OR] 2·23 [97·5% CI 1·16-4·31]; p=0·0060) and 151 (52%) of 291 patients versus 24 (32%) of 74 patients in BE HEARD II (2·29 [1·22-4·29]; p=0·0032). In BE HEARD II, HiSCR50 was also met in the group who were administered bimekizumab every 4 weeks (77 [54%] of 144 vs 24 [32%] of 74 with placebo; 2·42 [1·22-4·80]; p=0·0038). Responses were maintained or increased to week 48. Serious treatment-emergent adverse events were reported in 40 (8%) patients in BE HEARD I and in 24 (5%) patients in BE HEARD II treated with bimekizumab over 48 weeks. The most frequently reported treatment-emergent adverse events to week 48 were hidradenitis in both trials, in addition to coronavirus infection and diarrhoea in BE HEARD I, and oral candidiasis and headache in BE HEARD II. One death was reported across the two trials, and was due to congestive heart failure in a patient with substantial cardiovascular history treated with bimekizumab every 2 weeks in BE HEARD I (considered unrelated to bimekizumab treatment by the investigator). No new safety signals were observed. Bimekizumab was well tolerated by patients with hidradenitis suppurativa and produced rapid and deep clinically meaningful responses that were maintained up to 48 weeks. Data from these two trials support the use of bimekizumab for the treatment of patients with moderate-to-severe hidradenitis suppurativa. UCB Pharma.