Serum osteocalcin levels are a marker of bone formation. In this study, bone and mineral metabolism in type I diabetes mellitus (DM) were investigated, and the changes related to diabetic microvascular complications were examined. Serum calcium (Ca), inorganic phosphate (P), osteocalcin (OC) and parathyroid hormone (PTH) levels were measured in 42 type I diabetic subjects. Diabetics were subdivided into those with or without complications. Age and sex-matched control subjects were used for comparisons with the diabetic groups. Serum P and PTH levels were not different from those of controls. Serum Ca levels were significantly increased (p < 0.001) although the values were within the normal range. OC levels were significantly lower in the complicated (retinopathy and/or protenuria) diabetic group (p < 0.005). In Type I diabetes mellitus, the serum OC level is influenced by the presence of microvascular complications.