Background: Hidradenitis suppurativa (HS) is a chronic, painful skin disease characterized by abscesses, nodules, and draining fistulas in the axilla and groin of young adults. Objective: To evaluate the efficacy and safety of adalimumab, an anti-tumor necrosis factor-αantibody, in patients with moderate to severe HS. Design: Phase 2, parallel, randomized, placebo-controlled trial consisting of a blinded 16-week period (period 1) and an open-label 36-week period (period 2). All study personnel, investigators, and patients remained blinded to treatment group throughout the study. (ClinicalTrials.gov: NCT00918255) Setting: 26 academic and private practice medical centers in the United States and Europe. Patients: 154 adult patients with moderate to severe HS who were unresponsive or intolerant to oral antibiotics. Intervention: Patients were assigned in a 1:1:1 ratio to adalimumab, 40 mg/wk; adalimumab, 40 mg every other week (EOW); or placebo. All patients received adalimumab, 40 mg EOW, at the beginning of period 2 but switched to weekly dosing if the response was suboptimal (HS Physician's Global Assessment [PGA] score of moderate or worse) at weeks 28 or 31. Measurements: The primary outcome measure (clinical response) was the proportion of patients achieving an HS-PGA score of clear, minimal, or mild with at least a 2-grade improvement relative to baseline at week 16. Results: At week 16, 3.9% of placebo patients (2 of 51), 9.6% of EOW patients (5 of 52), and 17.6% of weekly patients (9 of 51) achieved clinical response (EOW vs. placebo strata-adjusted difference, 5.6% [95% CI,- 4.0% to 15.3%]; P = 0.25; weekly vs. placebo strata-adjusted difference, 13.7% [CI, 1.7% to 25.7%]; P = 0.025). Serious adverse event rates were 3.9%, 5.8%, and 7.8% for placebo, EOW, and weekly patients, respectively (EOW vs. placebo difference, 1.8% [CI,- 6.4% to 10.1%]; weekly vs. placebo difference, 3.9% [CI,- 5.2% to 13.0%]). Significantly greater improvements in patient-reported outcomes and pain were seen in the weekly dosing group than in the placebo group. A decrease in response was seen after the switch from weekly to EOW dosing in period 2. Limitations: Weeks 16 to 52 of the study were open-label. The study was not powered to assess the risk for known serious adverse effects of adalimumab, such as tuberculosis, other serious infections, and demyelinating disorders. Conclusion: Adalimumab dosed once per week alleviates moderate to severe HS. Primary Funding Source: Abbott Laboratories.