Summary Vitamin D deficiency reduces insulin secretion and still occurs in East London Asians in whom the prevalence of diabetes mellitus is at least four times that of Caucasians. Vitamin D status was assessed in 44 of 65 non-diabetic subjects 'at risk' of diabetes (spot blood glucose level > 6.0 mmol/1 < 2 h post cibum, or > 4.6 mmol/1 > 2 h post cibum on two separate occasions) and in 15 of 60 age and sexmatched 'low-risk' control subjects who attended for oral glucose tolerance test (OGTT) after screening of 877 omnivorous subjects not known to have diabetes. It was found that 95 % of at-risk and 80 % of lowrisk subjects were vitamin D deficient (serum 25-hydroxy-vitamin D < 11 ng/ml). Diabetes was present in 16, impaired glucose tolerance in 12 and normoglycaemia in 19 at-risk subjects, impaired glucose tolerance in 2, and normoglycaemia in 13 low-risk subjects. Correlations of 30-min OGTT blood glucose, specific insulin and C-peptide levels with 25-hydroxy-vitamin D concentrations in 44 at-risk subjects were -0.31 (p = 0.04), 0.59 (p = 0.0001) and 0.44 (p = 0.006). In 15 'not-at-risk' subjects 30-min OGTT specific insulin and C-peptide levels correlated with 25-hydroxy-vitaminD, r=0.39 (p = 0.04) and 0.16 (p = 0.43), respectively. Serum alkaline phosphatase concentration was higher in at-risk than not-at-risk subjects (59.6 vs 46.5 IU/1, p = 0.012); corrected calcium concentrations were comparable (2.38 vs 2.39mmol/1, p=0.7). .0001] and 3.6 + 1.8 to 13.5 + 7.4 ng/ml [p = 0.0001], (but not to low-risk group values of 179+89mU/1, 2.7 + 1.14 pmol/ml and 8.16 + 6.4 ng/ml), respectively. Both total serum alkaline phosphatase and corrected calcium concentrations rose following vitamin D treatment in the at-risk subjects by 11.1 + 8.22 (from 44 to 55IU/1) and 0.15+0.18, (2.43 to 2.57 mmol/1), respectively (p = 0.004). Abnormal glucose tolerance was unchanged by vitamin D treatment. The value of early and sustained repletion with vitamin D in diabetes prophylaxis should be examined in communities where vitamin D depletion is common. [Diabetologia (1995[Diabetologia ( ) 38: 1239[Diabetologia ( -1245