We reviewed empirical evidence regarding whether mostly heterosexual exists as a sexual orientation distinct from two adjacent groups on a sexual continuum--exclusively heterosexual and substantially bisexual. We addressed the question: Do mostly heterosexuals show a unique profile of sexual and romantic characteristics that distinguishes them as a separate sexual orientation group? We found sufficient data in four areas to support an affirmative answer. Individuals who acknowledged a mostly heterosexual orientation were distinct from adjacent sexual orientation groups in having a small degree of same-sex sexual and/or romantic attraction and, occasionally, same-sex behavior; constituted a substantial prevalence in the population; were relatively stable in their orientation over time; and reported that this sexual identity was subjectively meaningful to them. Findings suggested that self-identification as mostly heterosexual or an acknowledgment of slight same-sex sexuality increases during the teenage years, peaks around the early twenties (somewhat sooner for men than women), and remains relatively high during young adulthood. Limited evidence suggested that prevalence is lower among older participants. These findings have implications for our conceptualization of sexual orientation as a continuum, the nature of sex differences in sexuality, developmental changes in sexuality, biologically based assessments of sexual orientation, and an etiological theory of mostly heterosexuality. (Contains 1 table.)