A long-term study of 63 extremely obese persons with disturbed glucose tolerance showed a further deterioration of glucose tolerance over 10 years. In 9 out of 23 patients manifest diabetes mellitus developed despite partial weight reduction (greater than or equal to 10% of the initial weight excess). Out of 14 patients who did not lose weight during this time even 11 developed manifestations of diabetes. All patients who were in different weight groups at the 5- and 10-year follow-up showed a deterioration in glucose tolerance after 10 years. In the individual groups there were only minimal differences in other characteristics apart from the different weight behaviour.