The aim of the present study was to examine whether high or low levels of platelet monoamine oxidase (MAO) activity were associated with an increased risk of winter seasonal affective disorder (SAD) or of developing characteristic vegetative symptoms during episodes of the disorder. We also investigated the relationship between MAO activity and the Global Seasonality Scale (GSS), a measure of seasonal variation in sleep length, social activity, mood, weight, appetite, and energy level. Patients with SAD (n = 49), patients with subsyndromal SAD (n = 11), and normal volunteers (n = 25) participated in the study. We found significantly higher levels of platelet MAO activity in females but did not observe significant differences across age groups or between groups of patients tested in different seasons or mood states. MAO activity (whether high or low) was not associated with a significant increase in risk of SAD or of developing hypersomnia, hyperphagia, or carbohydrate craving during episodes of winter depression. We found no significant relationship between GSS and MAO activity. Patients who had made suicide attempts during an episode of SAD had significantly lower mean levels of platelet MAO activity than other patients.