In a study extending over five years 35 of 70 excessively overweight patients with subclinical diabetes mellitus who had not significantly changed their weight, demonstrated a further deterioration in glucose tolerance, and manifest diabetes occurred in ten. In the remaining 35 patients who had achieved 20% weight reduction from the initial level glucose tolerance had become normal. The two groups were similar as to age, initial weight, and original degree of glucose intolerance. There was a significant correlation between loss of weight and increased glucose tolerance. No such correlation occurred with subgroups of patients whose initial overweight was more than +100 relative percentage and who had been over 50 years old at the start of the study.