Interleukin-1 has been implicated as a mediator of recurrent pericarditis. The efficacy and safety of rilonacept, an interleukin-1α and interleukin-1β cytokine trap, were studied previously in a phase 2 trial involving patients with recurrent pericarditis.We conducted a phase 3, multicenter, double-blind, event-driven, randomized-withdrawal trial of rilonacept in patients with acute symptoms of recurrent pericarditis (as assessed on a patient-reported scale) and systemic inflammation (as shown by an elevated C-reactive protein [CRP] level). Patients presenting with pericarditis recurrence while receiving standard therapy were enrolled in a 12-week run-in period, during which rilonacept was initiated and background medications were discontinued. Patients who had a clinical response (i.e., met prespecified response criteria) were randomly assigned in a 1:1 ratio to receive continued rilonacept monotherapy or placebo, administered subcutaneously once weekly. The primary efficacy end point, assessed with a Cox proportional-hazards model, was the time to the first pericarditis recurrence. Safety was also assessed.A total of 86 patients with pericarditis pain and an elevated CRP level were enrolled in the run-in period. During the run-in period, the median time to resolution or near-resolution of pain was 5 days, and the median time to normalization of the CRP level was 7 days. A total of 61 patients underwent randomization. During the randomized-withdrawal period, there were too few recurrence events in the rilonacept group to allow for the median time to the first adjudicated recurrence to be calculated; the median time to the first adjudicated recurrence in the placebo group was 8.6 weeks (95% confidence interval [CI], 4.0 to 11.7; hazard ratio in a Cox proportional-hazards model, 0.04; 95% CI, 0.01 to 0.18; P<0.001 by the log-rank test). During this period, 2 of 30 patients (7%) in the rilonacept group had a pericarditis recurrence, as compared with 23 of 31 patients (74%) in the placebo group. In the run-in period, 4 patients had adverse events leading to the discontinuation of rilonacept therapy. The most common adverse events with rilonacept were injection-site reactions and upper respiratory tract infections.Among patients with recurrent pericarditis, rilonacept led to rapid resolution of recurrent pericarditis episodes and to a significantly lower risk of pericarditis recurrence than placebo. (Funded by Kiniksa Pharmaceuticals; RHAPSODY ClinicalTrials.gov number, NCT03737110.).