This study quantified changes in self-reported sexual orientation from adolescence to early adulthood, and whether childhood gender nonconformity (GNC) predicted sexual orientation changes. Youth (2,678 boys and 3,359 girls; 96.09% ethnically White) from the Avon Longitudinal Study of Parents and Children (ALSPAC) were included. Self-reported sexual orientation was measured using sexual attraction (5-point scale) at ages 15.5, 21, and 23. GNC was measured via Preschool Activities Inventory at ages 2.5, 3.5, and 4.75 years. The prevalence of boys and girls who reported being gay/lesbian increased from 15.5 to 21 years old whereas the proportion of bisexuals was relatively stable for both sexes. Among boys, heterosexuality and being gay were equally stable and relatively more stable compared to bisexuality. Among girls, reporting being lesbian and bisexual were equally unstable and relatively less stable than heterosexuality. Girls reporting being lesbian were more likely to report changes in their sexual orientation than gay adolescent boys. The stability of being lesbian and bisexual among girls, and bisexuality among boys, increased over time. Overall, few people changed their self-reported sexual orientation between ages 21 and 23. GNC at 2.5 years, and changes in GNC from 2.5 to 4.75 years, predicted being lesbian/gay at 15.5, 21, and 23 years and changes from being heterosexual to lesbian/gay from 15.5 to 21 years in each sex. In conclusion, self-reported sexual orientation from adolescence to young adulthood is relatively stable in males compared to females, and childhood GNC is a predictor of any, albeit small, sexual orientation changes.