Puerperal and menstrual psychoses are both uncommon disorders and the occurrence of both in individual patients suggests the possibility of a common underlying pathogenesis. In this paper two cases are reported, the literature is reviewed and a unifying etiological hypothesis is postulated in which precipitous reductions in the brain estrogen environment precipitate episodes of psychosis in predisposed individuals. In the case of puerperal psychosis, estrogen cascade follows a lengthy period of sustained high brain estrogen environment; in menstrual psychosis, it is postulated that the occurrence in at least some cases of anovulatory menstrual cycles, wherein high levels of relatively unopposed estrogens are maintained until the next ovulatory cycle, play a role in priming the central nervous system prior to premenstrual estrogen cascade. Further research in this area using more sensitive techniques to follow hormonal fluctuation and mental state is called for.