The vitamin D status of IDDs was studied in 3 groups of patients who were treated for several months with (i) conventional insulin therapy (group I, n = 17, HbA1 = 10.1 +/- 0.5%); (ii) continuous subcutaneous insulin infusion (CSII, group II, n = 11, HbA1 = 8.9 +/- 0.6%); and (iii) continuous intraperitoneal insulin infusion (CPII, group III, n = 13, HbA1 = 8.0 +/- 0.4%). In all patient groups the plasma concentration of vitamin D metabolites were within normal range. However plasma 25 OH D (ng/ml) was significantly lower in groups I (13.0 +/- 0.8, P less than 0.01) and II (12.5 +/- 1.5, P less than 0.02) than in group III: 22.1 +/- 2.3 (normal range 7-27). Plasma 24,25-(OH)2D (ng/ml) was positively correlated to plasma 25 OH D and was significantly decreased in groups I (1.5 +/- 0.2, P less than 0.05) and II (1.4 +/- 0.2, P less than 0.05) compared with group III: 2.3 +/- 0.3. No significant differences were found in plasma 1,25-(OH)2D between the three groups of diabetics. Plasma PTH was similar in the three groups. The same differences in plasma 25 OH D were observed between the patients treated with CPII and 15 subcutaneously treated patients matched for diabetic control (HbA1 less than 10 per cent). The present results seem to indicate that insulin might have a stimulatory effect on the hepatic 25 hydroxylase activity.