The oral glucose tolerance test (oGTT) was performed twice in patients under long-term lithium treatment. Blood glucose and plasma insulin were determined. The oGTT results were evaluated by three criteria (Köbberling-Creutzfeldt, WHO, and Epidemiological Study Group of the European Diabetes Association) and were compared to two representative reference studies from normal populations. The frequency of impaired glucose tolerance in the patients was three times higher than expected on the basis of the studies on normal populations. The variability of the oGTT curves between the first and second tests as well as the steepness of the time-course of the ‘insulinogenic index’ suggested mild disturbances of carbohydrate metabolism (mild diabetes) in some of the patients. It is considered unlikely that the impairment of glucose tolerance in the patients was a direct pharmacological effect of lithium salts. The possible role of age, sex, manic-depressive disease, additional medication, and particularly obesity in the effects of long-term lithium treatment on glucose tolerance is discussed. The authors suggest that the oGTT should be carried out periodically in long-term, lithium-treated patients over the age of 40 years in order to detect abnormalities in their carbohydrate metabolism.