Background:
Beginning in the late-1990s, rates of sexually transmitted infections (STI) started to increase in high-risk populations in the U.S., particularly MSM. While many reports have examined key sexual risk behaviors over time such as condom use, only a few have measured the number of recent sexual partners.

Objective:
To examine longitudinal trends in recent sexual partners reported by STI patrons from two urban clinics.

Method:
We separately measured and examined self-reported behavioral data from men and women attending two municipal STI clinics in San Francisco, CA (n=147,099), and Denver, CO (n=125,654) between 1995 and 2004.

Result:
In San Francisco, the mean number of partners in the past 2 months reported by (a) heterosexual women was 1.6 in 1995 and increased by 12.5% to 1.8 in 2004 (p < 0.0001), (b) heterosexual men was 1.7 in 1995 and increased by 18% to 2.0 in 2004 (p < 0.0001), and (c) MSM was 3.8 in 1995 and increased by 42% to 5.4 in 2004 (p < 0.0001). In Denver, the proportion of patients reporting two or more recent sexual partners in the past 4 months was (a) 13.1% in 1995 and 15.6% in 2004 for heterosexual women, an increase of 19% (p < 0.05), (b) 17.6% in 1995 and 21.6% in 2004 for heterosexual men, an increase of 23% (p < 0.0001), and (c) 43.3% in 1995 and 47.5% in 2004 for MSM, an increase of 10% (p < 0.005).

Conclusion:
Between 1995 and 2004, the number of recent sexual partners reported by STI patients increased significantly for all risk groups in both clinics.

Implications:
Ongoing prevention research is needed that sheds light on how increases in recent sexual partners impacts acquisition and transmission of STIs, particularly for MSM.