We assessed prevalence and risk factor data for men routinely screened for Chlamydia trachomatis and Neisseria gonorrhoeae in STD clinics in four US cities from May 1995-March 1999. Data were analysed separately for 'test-visits' (self-reported symptoms, clinical signs or sexual contact to an STD) and 'screen-visits' (STD screen only) for 32,595 men with 45,390 visits. Among test-visits in Seattle, Indianapolis and New Orleans, 8.7% (807/9285), 15.3% (1305/8519), and 10.1% (1551/15,296) of men were positive for C. trachomatis, and 10.2% (773/7543), 24.9% (2108/8478), and 30.4% (4746/ 15,629) for N. gonorrhoeae. Among screen-visits, 2.1% (88/4103), 7.3% (130/1790), and 5.6% (292/5183) of men were positive for C. trachomatis, and 1.8% (46/2576), 1.7% (31/ 1786), and 5.2% (274/5235) for N. gonorrhoeae. Positivity for screen-visits was particularly high among young men (15-24 years), and those reporting > 1 sex partner in the past 60 days. Substantial variation among sites in positivity warrants local determination of prevalence and risk factors to inform screening strategies.