OBJECTIVE:Plasma interleukin-1 beta may influence sepsis mortality, yet recombinant human interleukin-1 receptor antagonist did not reduce mortality in randomized trials. We tested for heterogeneity in the treatment effect of recombinant human interleukin-1 receptor antagonist by baseline plasma interleukin-1 beta or interleukin-1 receptor antagonist concentration. DESIGN:Retrospective subgroup analysis of randomized controlled trial. SETTING:Multicenter North American and European clinical trial. PATIENTS:Five hundred twenty-nine subjects with sepsis and hypotension or hypoperfusion, representing 59% of the original trial population. INTERVENTIONS:Random assignment of placebo or recombinant human interleukin-1 receptor antagonist × 72 hours. MEASUREMENTS AND MAIN RESULTS:We measured prerandomization plasma interleukin-1 beta and interleukin-1 receptor antagonist and tested for statistical interaction between recombinant human interleukin-1 receptor antagonist treatment and baseline plasma interleukin-1 receptor antagonist or interleukin-1 beta concentration on 28-day mortality. There was significant heterogeneity in the effect of recombinant human interleukin-1 receptor antagonist treatment by plasma interleukin-1 receptor antagonist concentration whether plasma interleukin-1 receptor antagonist was divided into deciles (interaction p = 0.046) or dichotomized (interaction p = 0.028). Interaction remained present across different predicted mortality levels. Among subjects with baseline plasma interleukin-1 receptor antagonist above 2,071 pg/mL (n = 283), recombinant human interleukin-1 receptor antagonist therapy reduced adjusted mortality from 45.4% to 34.3% (adjusted risk difference, –0.12; 95% CI, –0.23 to –0.01), p = 0.044. Mortality in subjects with plasma interleukin-1 receptor antagonist below 2,071 pg/mL was not reduced by recombinant human interleukin-1 receptor antagonist (adjusted risk difference, +0.07; 95% CI, –0.04 to +0.17), p = 0.230. Interaction between plasma interleukin-1 beta concentration and recombinant human interleukin-1 receptor antagonist treatment was not statistically significant. CONCLUSIONS:We report a heterogeneous effect of recombinant human interleukin-1 receptor antagonist on 28-day sepsis mortality that is potentially predictable by plasma interleukin-1 receptor antagonist in one trial. A precision clinical trial of recombinant human interleukin-1 receptor antagonist targeted to septic patients with high plasma interleukin-1 receptor antagonist may be worthy of consideration.