The present study examined the association between electrodermal activity (EDA) and season of birth in a sample of first-episode patients with schizophrenia, schizophreniform disorder, and affective disorder with psychotic features, and in a normal control group. Patients with schizophrenia who were born during the season of excess risk (January-April) were less responsive than those born during other times of the year. They had lower skin-conductance levels and fewer skin-conductance responses. No such effects were found in patients with schizophreniform or affective disorder, or in the normal subjects. When compared with the control group, winter-born schizophrenics showed significantly more evidence of hyporesponsivity. In contrast, nonwinter-born patients did not differ from normal subjects in skin-conductance level or number of skin-conductance responses. Schizophreniform patients born during the other seasons of the year were more likely to be hyporesponsive. The above results provide supporting evidence for the validity of the season of birth phenomenon. We hypothesize that a viral infection, or some other perinatal complication associated with winter and early spring births, leads to temporal lobe damage and consequent dyregulation of electrodermal activity in patients with schizophrenia.