The objective was to evaluate the prevalence and severity of osteopenia in patients with uncomplicated insulin-dependent diabetes mellitus (IDDM) and to obtain more information on the pathophysiology of diabetic osteopenia. In 35 patients with uncomplicated IDDM (21 men and 14 women; age 37.6±9.9 yr; duration of disease 8.5±3.5 years) bone mineral density was measured by dual energy X-ray absorptiometry (DEXA). In addition, markers of bone formation [plasma insulin-like growth factor I (IGF-I), serum alkaline phosphatase (ALP), serum bone alkaline phosphatase (BAP) and serum osteocalcin] and bone resorption [urinary excretion of calcium and of the cross-linked N-telopeptide of type 1 collagen, both corrected for the excretion of creatinine] were measured in the diabetic patients and in 33 healthy controls, matched for sex, age, height, weight and body mass index (BMI). In 67% of the diabetic men and 57% of the diabetic women osteopenia of the femoral neck and/or the lumbar spine (T-value ≤−1 SD) was present. Fourteen percent of the male patients, but none of the female patients, met the criteria for osteoporosis (T-value ≤−2.5 SD). In the whole group of diabetic patients the mean plasma IGF-I level tended to be lower (p<0.10) as compared to that in the controls. In the diabetic patients with femoral neck osteopenia, the mean plasma IGF-I level was significantly lower (p<0.05) than in those without osteopenia at this site. There were no differences in the mean serum ALP, BAP and osteocalcin levels between the diabetic patients and the controls, nor between the diabetic patients with and without femoral neck osteopenia. Considering only the male diabetic patients, significantly lower mean plasma IGF-I (−26%), serum ALP (−24%) and serum osteocalcin (−38%) levels were present in the patients with femoral neck osteopenia than in those without osteopenia at this site, suggesting lowered bone formation. The bone resorption markers were similar in all (sub)groups of diabetic patients and not different between diabetic patients and controls. Bone mineral density (BMD) did not correlate with plasma levels of glycosylated hemoglobin (HbA_1c). BMD values were not related to any of the bone resorption or formation markers, except for plasma IGF-I both in the femoral neck (r=+0.38, p=0.026) and the lumbar spine (r=+0.34, p=0.043). Our data demonstrate that at least in male patients with IDDM, osteopenia is the consequence of a lowered bone formation with a predominance of bone resorption over formation.