Objective: The objective of this study was to measure the prevalence of Chlamydia trachomatis (CT) infection among men in clinical and nonclinical settings across the United States. Goal: The goal of this study was to obtain data to inform recommendations regarding male CT screening. Study: The authors conducted a cross-sectional study of CT prevalence among adolescent and adult men in 4 U.S. cities (Baltimore, Denver, San Francisco, and Seattle). CT was detected using urine-based testing, and prevalence was calculated for first testing event. Results: Over 23,000 men were tested for CT over a 31/2-year period. The majority (96%) were asymptomatic. Overall, prevalence was 7% and varied significantly between cities (range: Seattle, 1%; Baltimore, 12%), by age (peak prevalence at age 20-24 years, 9%), and between venues where CT testing was offered. Conclusions: At 7%, the prevalence of CT is moderately high among men opportunistically tested in nonclinical and clinical settings.