Previous studies have indicated an association between childhood sexual abuse (CSA) and adult sexual risk behaviour among women and among men who have sex with men (MSM). However, no studies to date have tested the hypothesis that a history of CSA predicts sexual behaviour carrying risk of transmission of HIV to others, i.e. in a known HIV-positive cohort. The present study tested this hypothesis among a sample of 456 HIV-positive MSM recruited from community venues in New York and San Francisco. CSA history was found to be significantly associated with past (in the last 90 days) unprotected anal sex acts, both insertive (33% versus 20%, p < 0.05) and receptive (43% versus 27%, p < 0.02), with partners of HIV-negative or unknown serostatus. Further, several potential mediators of this effect were tested, and three found to be predicted by CSA history. Each of these potential mediators was associated with sexual risk behaviour, but differentially: anxiety and hostility were significantly associated with insertive acts, while anxiety, hostility and suicidality were associated with receptive acts. Mediation analyses supported the hypothesis that these factors significantly (albeit partially) accounted for the association of CSA with receptive anal intercourse. Nonsignificant mediation effects were found for insertive sex, suggesting the operation of unmeasured mediating variables. These results highlight the importance of mental health services for individuals who have been sexually abused, both for personal and for public health benefit, and also indicate a need for further research into mediators of CSA effects on transmission-related behaviour.