This 4-year longitudinal study investigates 3 hypothesized effects of the pubertal transition on the psychological distress of adolescent girls (N = ca. 200): early maturation, the impact of heterosexual versus same-sex friends, and prepubertal vulnerabilities, such as early emotional distress and father hostility. Building on Caspi and Moffitt's study of girls' delinquency, this study found that early maturing girls experienced significantly higher levels of psychological distress compared to their on-time and late maturing age-mates. In line with prior studies of girls' delinquency (Caspi, Lynam, Moffitt, & Silva, 1993; Magnusson, 1988), early maturing girls also experienced higher levels of distress at the tenth grade as reported by fathers, mothers, and adolescents when they were associated with mixed- rather than same-sex friends during the seventh through ninth grades. Finally, early maturing girls were more vulnerable to prior psychological problems, deviant peer pressures, and fathers' hostile feelings when compared to on-time and late maturing peers.