The increasing evidence that bipolar and unipolar affective disorders have different biological etiologies and courses of illness has been associated with an intensifying interest in specific treatment regimens for both disorders during the last decade. In this context, the question arose whether antidepressants exert similar efficacy in the acute treatment of bipolar compared to unipolar depression. Although the clinical impression does not indicate substantial differences in the efficacy of antidepressants between these groups of patients, empirical databases concerning this topic are rare. The present study compared the efficacy of antidepressants in 50 unipolar and 50 bipolar depressed inpatients (ICD-9 criteria) under naturalistic treatment conditions. Both groups of patients were matched for age, gender and duration of illness. Clinical assessments of status at the time of admission and at discharge were used to rate response to antidepressant treatment. Analyses of the data revealed that both groups of patients needed the same time for treatment response and did not show any significant differences in outcome measures at discharge. These findings do not concur with the hypothesis formulated by some experts in the field of affective disorders that antidepressants are less effective in the acute treatment of bipolar depressed patients compared to unipolar depressed patients.