Background:
The rate of sexual partnership formation is a primary determinant of sexually transmitted infection (STI) transmission dynamics.

Objective:
To estimate age-specific partnership formation rates among men who have sex with men (MSM).

Method:
The Urban Men's Health Study (UMHS) is a 1996-1998 random-digit-dial survey of 2,881 MSM age ≥18 in 4 U.S. cities. We analyzed data from 2,553 men who answered questions regarding their number of sex partners since age 18, in the past 5 years, and in the past year. For intervals defined by these data, we assumed that partnerships were evenly distributed and calculated age-specific formation rates, stratifying these rates by levels of sexual activity (“low” ≤3 partners in past year or “high” ≥4 partners in past year).

Result:
The partnership formation rate for MSM with “low” activity was 6.9 (95% CI 5.5-8.4) partners per year at age 20, peaked at 8.6 (6.7-10.5) at age 28, and decreased only slightly to 6.3 (4.8-7.9) at age 50. The rate for MSM with “high” activity increased from 20.8 (17.6-24.0) at age 20 to its peak of 25.7 (19.9-31.5) at age 40, and then decreased steadily to 16.8 (12.1-21.5) at age 50. MSM born between 1940-1959 realized substantial declines in rates in the late-1980s/early-1990s, while men born 1960 and later had stable rates which were lower than earlier cohorts. Although the shape of partnership formation rate curves by age was similar between cities, the rates themselves varied, with the highest rates seen in San Francisco and the lowest in Chicago.

Conclusion:
The high rate of STI among MSM is likely partially a result of the sustained, relatively high rate of partnership formation observed in the population throughout adulthood.

Implications:
Use of the estimates we describe in mathematical models could help define STI transmission dynamics in MSM.