This study was undertaken to explore patterns of sexual orientation in a representative sample of Minnesota junior and senior high school students. The sample included 34,706 students (grades 7 through 12) from diverse ethnic, geographic, and socioeconomic strata. Five items pertaining to sexual attraction, fantasy, behavior, and affiliation were embedded in a self-administered survey of adolescent health. Overall, 10.7% of students were "unsure" of their sexual orientation; 88.2% described themselves as predominantly heterosexual; and 1.1% described themselves as bisexual or predominantly homosexual. The reported prevalence of homosexual attractions (4.5%) exceeded homosexual fantasies (2.6%), sexual behavior (1%), or affiliation (0.4%). Gender differences were minor; but responses to individual sexual orientation items varied with age, religiosity, ethnicity, and socioeconomic status. Uncertainty about sexual orientation diminished in successively older age groups, with corresponding increases in heterosexual and homosexual affiliation. The findings suggest an unfolding of sexual identity during adolescence, influenced by sexual experience and demographic factors.