This study examined the relationship between birth order and birth interval and deviant sexual preferences. It was hypothesized, based on the immunoreactivity hypothesis, that a larger number of older brothers and a shorter birth interval between the offenders and their next older brothers would be associated with more deviant responding. Participants of the study consisted of 168 male inmates serving a Federal sentence in a Canadian medium-security prison. Information on birth order and birth interval was obtained from various sources that included: psychological and psychiatric reports, institutional files, and interviews. The results of the study were consistent with the hypothesis that some sexual preferences can be affected by maternal immunoreactive influences on the development of the male fetal brain. In addition, it supported the idea that older sisters were not involved in the development of deviant sexual preferences. Offenders with shorter birth intervals from their older brothers had a greater interest in nonsexual, violent activities with children and adults, and offenders with longer birth intervals to their next younger brother showed greater sexual deviance on three indices. Tables and references