The purpose of this study was to assess factors associated with the use of family planning at first sexual intercourse among young adults aged 15 to 24 in urban Ecuador. The study population consisted of 1443 young adults (494 females and 949 males) in the cities of Quito and Guayaquil, interviewed by the 1988 Ecuador Young Adult Reproductive Health Survey, who reported having experienced consensual sexual intercourse. Approximately 11% of females and 15% of males reported using contraception at first intercourse. Binary logistic regression was performed to assess jointly the effect of multiple factors on contraceptive use at first intercourse. The regression model was first run on the entire study population and then separately for males and females. In the overall population, the following variables were significantly related to using family planning at first sex: being male; being from Guayaquil; older age; father's completion of secondary school. Having lost one's virginity to a prostitute was significantly associated with non-use of family planning. Males were 3·6 times more likely than females to use family planning during their first sexual intercourse. For each year older a young adult was at first sex, his or her odds of using family planning was multiplied by a factor of 1·3. Twenty-eight per cent of males in this study experienced their first sexual intercourse with a prostitute, and these young men were highly unlikely to use family planning. A male who experienced first intercourse with his girlfriend was more than five times as likely to use contraception than a male who lost his virginity to a prostitute.