The aim of this study was to evaluate the effect of vitamin D3 supplementation on insulin secretion and insulin resistance. Ten females with type 2 diabetes being treated with oral hypoglycaemic agents and with normal serum and urine calcium levels were enrolled in the study. The study was conducted in March, when levels of vitamin D are lowest in our region. The level of plasma 25(OH)D was measured (normal range in winter 25-120 nmol/l). The first (FPIS) and second (SPIS) phases of insulin secretion were studied during IVGTT. Peripheral insulin resistance was measured. A group of 17 age- and BMI-matched females with normal glucose tolerance served as a control group. The diabetic patients were treated with cholecalciferol 1332 IU daily for one month. The mean plasma 25(OH)D level was 35.3 +/- 15.1 nmol/l at baseline, 70% of patients being vitamin D deficient. After one month of treatment with vitamin D3, the plasma 25(OH)D level increased by a mean of 75.8%; 70% of the patients achieved normal vitamin D levels. FPIS increased significantly by 34.3%, while the change in SPIS of 20.4% was not significant (p > 0.8). We found a significant correlation between the change in FPIS and the change in 25(OH)D level after vitamin D3 supplementation (p < 0.018). The results showed a decrease of 21.4% in insulin resistance after one month, but the change was not significant. Bearing in mind that the main defects in type 2 diabetes mellitus are reduced FPIS and insulin resistance, and the favourable effect vitamin D3 had on them, we suggest vitamin D3 deficiency may at least partly contribute to the impairment of insulin secretion and probably of insulin action. Our results suggest that vitamin D3 supplementation could be an element in the complex treatment of type 2 diabetes mellitus during the winter.