PURPOSE: The purpose of this paper is to define a typology that encompasses the full range of adolescent heterosexual behavior; to compare the usefulness of the new typology with that of the traditional dichotomy of "sexually active"/"sexually inactive" for understanding sexual behavior among adolescents; and to determine the implications of the new typology for the design and implementation of HIV prevention programs targeting adolescents. METHODS: Detailed face-to-face interviews were conducted with a cross-sectional sample of 907 mothers and their adolescents, ages 14-17 years, recruited from public high schools in Alabama, New York, and Puerto Rico. Information from the adolescent survey on precoital sexual behaviors and STD/HIV sexual risk and risk reduction behaviors was examined. A typology of adolescent heterosexual experiences was constructed using four behavioral dimensions. RESULTS: Ninety-nine percent (n = 894) of the sample was classified into one of the five patterns of sexual experience: Delayers, Anticipators, One-timers, Steadies, and Multiples. Among the participants who were not sexually active, precoital behaviors differed significantly between the 22% who anticipated initiating sexual intercourse in the next year (Anticipators) and those who did not (Delayers). Among those traditionally classified as "sexually active", One-timers and Steadies were significantly older when they first had penile-vaginal intercourse than those who had multiple partners. One-timers were more likely to use condoms than Steadies or Multiples, and only Multiples reported previous STDs. CONCLUSION: A typology that defines a range of adolescent heterosexual experiences was developed, and it was possible to classify 99% of our sample. The traditional dichotomy between "sexually active" vs. "not active" hides important behavioral intentions and sexual practices. These differences must be taken into account in the development and implementation of HIV prevention programs that target adolescents.