OBJECTIVE Men who have sex with men (MSM) have higher rates of HIV and other sexually transmitted infections than women and heterosexual men. This elevated risk persists across age groups and reflects biological and behavioral factors; yet, there have been few direct comparisons of sexual behavior patterns between these populations. METHODS We compared sexual behavior patterns of MSM and male and female heterosexuals aged 18-39 using 4 population-based random digit dialing surveys. A 1996-1998 survey in 4 US cities and 2 Seattle surveys (2003 and 2006) provided estimates for MSM; a 2003-2004 Seattle survey provided data about heterosexual men and women. RESULTS Sexual debut occurred earlier among MSM than heterosexuals. MSM reported longer cumulative lifetime periods of new partner acquisition than heterosexuals and a more gradual decline in new partnership formation with age. Among MSM, 86% of 18- to 24-year-olds and 72% of 35- to 39-year-olds formed a new partnership during the previous year, compared with 56% of heterosexual men and 34% of women at 18-24 years, and 21% and 10%, respectively, at 35-39 years. MSM were also more likely to choose partners >5 years older and were 2-3 times as likely as heterosexuals to report recent concurrent partnerships. MSM reported more consistent condom use during anal sex than heterosexuals reported during vaginal sex. CONCLUSIONS MSM have longer periods of partnership acquisition, a higher prevalence of partnership concurrency, and more age disassortative mixing than heterosexuals. These factors likely help to explain higher HIV/sexually transmitted infections rates among MSM, despite higher levels of condom use.