Background Rate of population change gives an indication of the average economic prosperity in a geographic area that reflects changing conditions to its physical and social environment which may influence the STIs risk behaviours found. Men who buy sex from sex workers (clients) are at high-risk of STIs and key in the spread infection to the general population. This study analyses census data matched with survey data to examine the association between population change in U.S. metropolitan counties with clients.

Methods A sample of 385 metropolitan men embedded in a 1999–2000 national probability telephone survey was linked to 1990 and 2000 U.S. Census county population counts. County population change between 1990 and 2000 was measured categorically and as a transformed continuous variable to account for negative growth and its skewed distribution. Poisson regression was used to obtain prevalence ratio (PR).

Results The sample was 73% non-Hispanic white, median age of 36 years, 77% were currently married or living with a partner and 39% had completed at least four years of college. The prevalence of clients was 14.8% (CI 11.5–18.9%), differing significantly by the population relative change of the county of residence (P < 0.05). The lowest and highest prevalence of clients was observed among men residing in counties that loss population (6.0% (CI, 2.0–17.2) and grew rapidly above the national average (21.4%, CI, 13.5%–32.3%). Using the transformed continuous measure for population change, there was a statistically significant increase in the ratio of male clients with a unit increase in population growth above the national average (unadjusted PR, 0.2, CI, 0.1–0.5, P < 0.010).

Conclusion The relationship between rate of urbanisation in metropolitan areas and male clients provides support for demographic changes influencing level of STI risks in the population and may contribute to development of comprehensive STI prevention activities.