This study examines whether riskier sexual behavior or duration of sexual experience explains why women who become sexually active earlier in life have a higher prevalence of sexually transmitted disease (STD). Responses to a self-administered questionnaire on risk behavior from 4,342 single women attending Planned Parenthood clinics in Pennsylvania were analyzed. Logistic regression was used to control for years of sexual activity, race, and amount of education. Women who became sexually active between the ages of 10 and 14 years were almost 4 times more likely to report having 5 or more sexual partners in the past year (OR = 3.8; 95% CI = 2.6-5.6); 3 times more likely to report having sex with bisexual, intravenous drug-using, or human immunodeficiency virus (HIV)-infected men (OR = 3.5; 95% CI = 2.4-5.0); and twice as likely to report a history of STD within the last 5 years (OR = 2.3; 95% CI = 1.8-3.0) compared with women who became sexually active when they were 17 years of age or older. The analysis suggests that age at first intercourse is a useful marker for risky sexual behavior and history of STD.