Abstract: Objective: Premenstrual onset psychosis is a rare condition of unknown etiology for which no treatment trials have been conducted and whose existence as a definitive diagnosis continues to be debated. The literature includes individual case reports and small case series, leaving psychiatrists to make decisions about prescribing antipsychotic agents on a case-by-case basis. Moreover, researchers continue to debate the efficacy of antipsychotic agents in the treatment of premenstrual onset psychosis.Method: Case report.Results: We report the case of a 17-year-old female with recurrent premenstrual onset psychosis that was successfully treated with olanzapine monotherapy (20 mg/day).Conclusion: These findings may serve as a reminder to physicians to rethink the suitability of a more traditionally accepted diagnosis, including premenstrual exacerbation of bipolar disorder or schizophrenia, and the potentially important role of antipsychotic agents, especially prolactin-sparing ones, in premenstrual onset psychosis.