Several clinical trials have demonstrated that antiretroviral (ARV) drugs taken as pre-exposure prophylaxis (PrEP) can prevent HIV infection, with the magnitude of protection ranging from −49 to 86% (refs. ^,,,,,,,,,). Although these divergent outcomes are thought to be due primarily to differences in product adherence, biological factors likely contribute. Despite selective recruitment of higher-risk participants for prevention trials, HIV risk is heterogeneous even within higher-risk groups^,,. To determine whether this heterogeneity could influence patient outcomes following PrEP, we undertook a post hoc prospective analysis of results from the CAPRISA 004 trial for 1% tenofovir gel (n = 774 patients), one of the first trials to demonstrate protection against HIV infection. Concentrations of nine proinflammatory cytokines were measured in cervicovaginal lavages at >2,000 visits, and a graduated cytokine score was used to define genital inflammation. In women without genital inflammation, tenofovir was 57% protective against HIV (95% confidence interval (CI): 7–80%) but was 3% protective (95% CI: −104–54%) if genital inflammation was present. Among women who highly adhered to the gel, tenofovir protection was 75% (95% CI: 25–92%) in women without inflammation compared to −10% (95% CI: −184–57%) in women with inflammation. Immunological predictors of HIV risk may modify the effectiveness of tools for HIV prevention; reducing genital inflammation in women may augment HIV prevention efforts.