The response to lithium prophylaxis was assessed in a sample of bipolar patients subdivided into the following groups on the basis of the previous pattern of course of their illness: MDI (sequence mania-depression-free interval), DMI (sequence depression-mania-free interval), CC-LC (continuous circular course with long cycles), CC-RC (continuous circular course with rapid cycles), IRR (irregular course). A significant reduction of the mean number of morbid episodes and of the mean total morbidity during lithium treatment was observed only in patients with a previous MDI or IRR course. The percentage of responders to prophylaxis was significantly different among the five groups, and the difference could be mainly ascribed to the high response rate in the MDI group and the low response rate in the DMI and CC-RC groups. These results suggest that the classification of bipolar patients according to the previous pattern of course of their illness may be useful for the prediction of lithium response.