OBJECTIVE: To assess immediate antecedents of and seasonal variations in sexual intercourse among a focused population of medically indigent adolescent females in Houston, Texas. STUDY DESIGN: From June 2001 to March 2002 we solicited a convenience sample of adolescent females during their visit to a school-based clinic. The survey consisted of 34 multiple choice questions targeting immediate antecedents of sexual behavior. Demographic characteristics and survey responses were compared with t tests and ANOVA. The χ 2 goodness of fit test and test of homogeneity were used to assess seasonality. RESULTS: One hundred thirty-three teenagers completed our survey. Both African American and Hispanic teens were significantly more likely to report that their sexual debut occurred in June (p < 0.01 and = 0.02, respectively). More than 50% of those surveyed reported having sex either in their homes or in the homes of their partners. Doing "nothing" was the most common antecedent activity for sexual intercourse. There were no statistically significant differences in location or antecedent activity for first or most recent sexual intercourse between teens with and without prior pregnancy. CONCLUSION: Both the timing and location of sexual intercourse among the teens in this study suggest that unsupervised time may be a factor contributing to sexual activity in teens.