The cause of transsexualism remains unclear. The hypothesis that atypical prenatal hormone exposure could be a factor in the development of transsexualism was examined by establishing whether an atypical pattern of cognitive functioning was present in homosexual transsexuals. Possible activating effects of sex hormones as a result of cross-sex hormone treatment were also studied. Female-to-male and male-to-female transsexuals were compared with female and male controls with respect to spatial ability before and after treatment. The data were consistent with an organizing effect, but there was no evidence of an activating effect. Homosexual transsexuals, who prior to hormone treatment scored in the direction of the opposite sex, may have reached a ceiling in performance and therefore do not benefit from activating hormonal effects.