To evaluate the relationship between insulin and hormonal vitamin D system we compared the serum concentrations of 1.25 dihydroxyvitamin D (1.25 D) in a group of 56 patients with insulin dependent diabetes mellitus (IDDM) with those from a control population of 26 healthy individuals. We did not find overall significant differences in 1.25 D levels between diabetic patients and control individuals (32.6 +/- 12 vs 35.4 +/- 9 pg/ml), but there were differences between the different patient subgroups depending on the duration of the disease. The dose of insulin and the 1.25 D levels were linearly correlated (r = 0.3693, p = 0.0051). The patients with IDDM had normal 1.25 D levels. Exogenous insulin administration may have relevance for the interpretation of these results.