The aim of the present study was to examine possible associations between platelet monoamine oxidase (MAO) activity and primary dysthymic disorder. For that purpose we estimated the enzyme activity in 58 patients (15 males and 43 females) selected according to DSM-III-R criteria and in 61 healthy controls (30 males and 31 females). Platelet MAO activities were found significantly lower in the female patients compared to female controls. Moreover, the enzyme activities were found to be even lower in the female patients who had attempted suicide. These differences did not exist in the male population. We could not find any associations of MAO activity to the age of the patients, the age of onset, the duration of dysthymia, or HAM-D and SCL-90R scores. Our findings are consistent with the hypothesis that platelet MAO activity is a trait-dependent indicator of vulnerability to dysthymic disorder and suicidality in our female population.