To assess the influence of diabetes mellitus on bone metabolism, we measured skeletal mass in the forearms of 35 patients with juvenile diabetes on insulin and 101 stable patients with adult-onset diabetes, on diet alone, insulin, or oral hypoglycemic agents. There was a significant loss of bone mass in both juvenile and adult-onset diabetes (P less than 0.01) as compared to controls matched for age and sex. The decrease was already present in patients with diabetes of less than five years' duration. Bone loss and duration of the diabetes did not correlate; the greatest decrease in bone mass was observed in the patients receiving oral agents. These data are consistent with the hypothesis that the loss of skeletal tissue in diabetes reflects the underlying disease since it occurs early and is not related to severity as evidenced by the need for insulin, to duration, or to treatment with insulin or diet alone.