Vitamin D metabolites and vitamin D-binding protein were measured in the serum of nonketotic Bantu and Caucasian insulin-requiring diabetic subjects from Zaire and Belgium, respectively. In Caucasian diabetics, whether untreated (N = 18) or insulin treated (N = 26), no abnormalities were found. The Bantu diabetics (N = 20) were more insulin-deficient and had a poorer glucose control than the Caucasians. They presented, compared with Bantu controls, a significant decrease in the serum concentrations of 25-hydroxyvitamin D3 (26 +/- 10 vs. 35 +/- 14 micrograms/L, P less than .01), 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] (38 +/- 15 vs. 58 +/- 17 ng/L, P less than .001), and vitamin D-binding protein (303 +/- 55 vs. 356 +/- 41 mg/L, P less than .001). The decreased concentrations of vitamin D metabolites in the adult Bantu diabetic patients may be partly explained by a concomitant decrease in the concentration of vitamin D-binding protein, possibly due to insulin deficiency. The ratio between the molar concentrations of 1,25-(OH)2D3 and vitamin D-binding protein, used as an index of the free hormonal level, was also decreased, in association with a decreased serum calcium level. In conclusion, no abnormalities in vitamin D metabolism were found in Caucasian insulin-dependent diabetics, whereas low serum 1,25(OH)2D3 concentrations and hypocalcemia were found in poorly controlled Bantu diabetic subjects.