Bone mineral content was measured by photon absorptiometry in 215 insulin treated diabetic out-patients aged 7--70 years. As bone mass increases until the age of 20--25 years, patients were so selected that all remained within the same phase of bone mineral storage throughout the entire course of their diabetes. Other criteria for exclusion were diseases or drugs interfering with mineral metabolism and previous use of oral antidiabetic agents. As a group the patients demonstrated a bone mineral deficit of 9.8% compared with sex- and age-matched controls (P less than 0.001). Comparison between patients who had developed diabetes before the age of 20 years and after that of 25 years revealed deficits of 14% and 7%, respectively (P less than 0.001). Sex differences were not observed. The initiation of osteopenia seemed to coincide with the onset of clinical diabetes mellitus, and significantly reduced bone mineral content was observed after 2 years of diabetes (P less than 0.001). After 3--5 years the osteopenia appeared to attain a stable level.