Few studies examine the influence of body mass index (BMI) on sexual risk. The purpose of this study was to determine whether BMI among 704 young mothers (ages 14-25) related to STI incidence and sexual risk. We examined the effect of BMI groups (normal weight, overweight, and obese) at 6 months postpartum on STI incidence and risky sex (e.g., unprotected sex, multiple partners, risky and casual partner) at 12 months postpartum. At 6 months postpartum, 31% of participants were overweight and 40% were obese. Overweight women were more likely to have an STI (OR = 1.79, 95% CI = 1.11-2.89, P < .05) and a risky partner (OR = 1.64, 95% CI = 1.01-2.08, P < .05) at 12 months postpartum compared to normal weight women. However, obese women were less likely to have an STI than normal weight women (OR = .57, 95% CI = .34-.96, P < .01). BMI related to STI incidence and sexual risk behavior. Integrated approaches to weight loss and sexual risk prevention should be explored.