OBJECTIVE: To assess the effect of type 1 and type 2 diabetes and insulin treatment on bone mineral density (BMD) in middle-aged and elderly men and women. RESEARCH DESIGN AND METHODS: We measured BMD and evaluated known determinants of osteoporosis in 56 type 1 and 68 type 2 diabetic patients and 498 nondiabetic community control subjects. All patients, aged 52-72 years, developed diabetes after the age of 30 years (i.e., after achievement of peak bone mass) and were treated with insulin. BMD was measured at the proximal femur with dual-energy X-ray absorptiometry. RESULTS: Among both sexes, BMD values were significantly lower in type 1 diabetic patients than in type 2 diabetic patients or the control subjects. When adjusted for age and BMI, the differences between type 1 diabetic patients and control subjects remained essentially unchanged in both sexes, whereas the differences between type 1 and type 2 diabetic subjects were significant only in men. After further adjustments for confounding factors, the average BMD values were still lower in type 1 diabetic subjects than in type 2 diabetic subjects although with lesser significance. Past low-energy fractures were more common in type 1 diabetic women than in type 2 diabetic women. CONCLUSIONS: The lower BMD in type 1 versus type 2 diabetic patients and control subjects probably results from more rapid bone loss after the onset of type 1 diabetes. This cannot be explained by insulin treatment, which was prescribed for both types of patients. Because the causes of low BMD in type 1 diabetes are unknown, these patients should be evaluated for the risk of osteoporosis and related fractures and offered appropriate preventive measures.