Fasting blood sugar was determined in manic-depressive patients before the start of lithium treatment and at intervals during treatment for up to 6 years. The total exposure time to lithium was 495.5 years. Even though the patients gained significantly in age and body weight during the treatment, their mean blood sugar values remained the same as before treatment was started, and only 1 patient developed manifest diabetes. There is no reason to believe that long-term lithium treatment leads to increased risk of developing diabetes mellitus.