Although it is now more than 30 years since Leohard originally proposed the distinction between bipolar and monopolar (unipolar) forms of affective disorder, there have been relatively few studies which have investigated clinical features which may differentiate the depressed phase of bipolar disorder from unipolar depression. In this study we examined the value of a new scale for rating depressive mental state signs (the 'core' score system), and a large series of symptoms and risk factors, in distinguishing between 27 age and sex-matched pairs of bipolar and unipolar patients diagnosed as melancholic on several diagnostic criteria. In general, we found a marked similarity between the groups on clinical features of the depressive episode when allowance was made for multiple tests. Bipolar patients, however, had shorter episodes of depression and were less likely to demonstrate 'slowed movements' than unipolar subjects. There were also consistent trends on other items for psychomotor retardation to be less common and agitation to be more likely in the bipolar patients. At the least, these findings suggest that the widely-held belief that bipolar depressed patients typically have psychomotor retardation is not as clear-cut as has been previously described.