Platelet monoamine oxidase (MAO) activities were assessed in 82 patients, 57 females and 25 males, who were admitted to the medical ward of a general hospital after a suicide attempt. The enzyme activities were compared to the activities of healthy subjects, 35 females and 26 males. In addition, MAO activities were analyzed in relation to sex, psychiatric diagnosis, mode of attempt, drugs ingested, and previous attempts. Compared to normal controls, only female patients showed lower MAO activities. In the male population, lower activities were found in the subgroup of patients who had made previous attempts. In relation to diagnosis, analysis performed in the female population revealed lower MAO activities in the dysthymic and personality disorder, and not in the adjustment or major affective disorder subgroups. MAO activities were not related to the violent mode of attempt, the type of medication used, or the score in the Beck Suicidal Intent scale. The finding of low platelet MAO activities in dysthymic disorder, indicates the need for further studies of biological variables in this underdiagnosed and undertreated diagnostic group.