Population-based HIV/AIDS prevalence estimates among men who have sex with men (MSM) have been unavailable, but have implications for effective prevention efforts. Prevalent (living) Florida HIV/AIDS cases reported through 2006 (numerators) were stratified by race/ethnicity and HIV exposure category. Based on previous research, MSM populations were posited as 4-10% of all males aged > or =13 years in each subgroup (denominators). At the estimated lower and upper plausible bounds, respectively, HIV/AIDS prevalence per 100,000 MSM was significantly higher among black (8,292.6-20,731.4); Hispanic (5,599.5-13,998.7); and Asian/Pacific Islander, American Indian or multi-racial (4,942.6-12,356.8) MSM than among white MSM (3,444.9-8,612.3). HIV/AIDS prevalence among all MSM was 13.8-36.9 times that among all other males. Across 19 high-morbidity counties, MSM HIV/AIDS prevalence was highest among those in the most populous counties and highest among blacks. This methodology, adaptable by other states, facilitates calculation of plausible MSM HIV/AIDS prevalence to guide HIV prevention/care community planners and MSM.