Background: Increased risk for serious adverse outcomes has been associated with a history of childhood sexual abuse (CSA). Whether these risks are directly attributable to CSA rather than family background remains controversial. Methods: Structured psychiatric telephone interviews were conducted from February 1996 to September 2000 with both members of 1991 same-sex pairs (1159 female and 832 male pairs) from a young adult Australian volunteer twin panel (mean [SD] age, 29.9 [2.5] years). A binary composite CSA variable was constructed from responses to 5 component questions. The association between CSA and adverse psychosocial outcomes was examined, controlling for family background. Results: A history of CSA, reported by 16.7% of the women and 5.4% of the men, was more common among those reporting parental alcohol-related problems. Significantly increased risk was observed in women reporting a history of CSA for subsequently occurring major depression, suicide attempt, conduct disorder, alcohol dependence, nicotine dependence, social anxiety, rape after the age of 18 years, and divorce; most similar risks reached statistical significance in men. The greatest risks were associated with CSA involving intercourse. Childhood sexual abuse-negative twins (ie, those who denied having experienced CSA) from CSA-discordant pairs compared with other CSA-negative individuals had increased risk for many adverse outcomes suggesting correlated family background risk factors. Childhood sexual abuse - positive members (ie, those who reported having experienced CSA) of CSA-discordant pairs had significantly greater risk for all 8 examined adverse outcomes than their co-twins. Conclusions: Self-reported CSA was associated with increased risk for adverse outcomes, controlling for family background. Family background risk factors also were associated with adverse outcome risk. Discordant pair analysis seems to provide an effective means of controlling for family background risk factors.