Osteoporosis is well documented in type I diabetes, but its occurrence is controversial in type II diabetes. Microangiopathy is a major complication of type I and type II diabetes. We studied bone and microvascular changes in the Cohen diabetic rat, a unique nonobese model of noninsulin-dependent diabetes mellitus. The aim of this study was to find whether there is a temporal correlation between the onset of these two complications. The diabetic rats were divided into three groups (A, B, and C) according to duration of diabetes (2 months, 3 months, and 7 to 8 months, respectively). Trabecular bone area was assessed by computerized image analysis and microangiopathy by means of renal function tests, histologic examination of the kidneys, and ultrastructural measurement of the width of capillary basement membranes. Bone density of the distal femur and vertebra was significantly reduced in the diabetic rats relative to the control rats in all three groups (Group A femur: 11.5 +/- 1.6% versus 21.8 +/- 3.0%, p < 0.02; Group A vertebra: 15.9 +/- 1.6% versus 28.5 +/- 2.0%, p < 0.02; Group C femur: 7.9 +/- 1.1% versus 29.6 +/- 3.5%, p < 0.001; Group C vertebra: 11.4 +/- 0.7% versus 37.1 +/- 1.9%, p < 0.002). Renal function tests were normal in the Group A diabetic rats and there was marked albuminuria in the Group C diabetic rats. Histologic changes in the kidneys were seen only in the Group C diabetic rats. Five of 15 Group C diabetic rats showed no albuminuria or histologic evidence of kidney damage. The bone density in this subgroup was reduced relative to controls to the same degree as that of the rats with renal damage. There was no evidence of capillary basement membrane thickening in the Group A diabetic rats. Our findings indicate that in the Cohen diabetic rat, osteoporosis precedes the onset of microangiopathy. Microangiopathy probably does not play an important role in the pathogenesis of osteoporosis in this animal model.