Objective. To assess the prevalence, clinical manifestations, associated genital infections, and HLA associations of reactive arthritis (ReA) among patients attending an urban sexually transmitted diseases (STD) clinic. Methods. Using a standardized questionnaire, 271 consecutive adults, primarily black, with possible or proven Chlamydia trachomatis genital infection were screened for symptoms of ReA. A followup questionnaire was administered 6 weeks later by mail. Patients who reported at least I symptom were evaluated by a rheumatologist. HLA-B typing was performed on patients with objective ReA features. Results. Nine of 217 patients (4.1%) with genital infection/inflammation had objective ReA features. Chlamydial or nongonococcal STD syndromes were diagnosed in 8 of these 9 patients (88%). Genital infection/inflammation was asymptomatic in 78% of patients with ReA features. HLA-B27 or other B7-cross-reactive group antigens were not associated with the occurrence of ReA. Conclusion. Nongonococcal genital infections, often asymptomatic, can trigger a relatively mild ReA in a larger number of exposed patients than previously thought, irrespective of the individual's HLA status.