Sixty-nine patients with type I diabetes mellitus were followed for from 1-4 yr (mean, 3 yr). Their overall growth, as measured by height and weight, was normal; however, repeated measurements of their bone mass using photon absorptiometry and radiogrammetry showed that, relative to normal subjects, the patients had a persistent bone deficit throughout the course of the study. This deficit was not attributable to bone width, which was normal. On the average, the magnitude of the deficit did not change with time; furthermore, an individual's rate of change in bone mass deficit during the study was not correlated with the patient's glucose control, as measured by hemoglobin A-1 or fasting blood glucose levels. Initial levels of serum ionized calcium and magnesium were decreased in the patients with diabetes. During the study, the mean level of ionized calcium increased, but that of magnesium decreased further, compared to the initial values. In a group of 19 patients with newly diagnosed diabetes, bone mass was found to be significantly below normal among the girls, but not among the boys.