OBJECTIVE This study aims to compare microvascular endothelial function between vitamin D-deficient and nondeficient groups of patients with diabetic nephropathy. Serum levels of the inflammatory marker high-sensitivity C-reactive protein (hsCRP) were also measured. METHODS This prospective cross-sectional study involved 70 patients with diabetic nephropathy; 40 were categorized into the group with nondeficient serum 25-hydroxyvitamin D levels [25(OH)D >50 nmol/l], whereas 30 patients were categorized to the group with deficient serum 25(OH)D (<50 nmol/l). Microvascular endothelial function was determined using laser Doppler fluximetry and the process of iontophoresis. Acetylcholine and sodium nitroprusside were used to determine endothelium-dependent and independent vasodilatation. RESULTS Mean age of patients was 56.7 ± 3.8 years; 50 were men, whereas 20 were women. Mean serum 25(OH)D in the vitamin D-nondeficient group was 69.4 ± 2.9 nmol/l; the level in the vitamin D-deficient group was 42.1 ± 1.3 nmol/l, P < 0.001. Endothelium-dependent vasodilatation was lower in the vitamin D-deficient group compared with the vitamin D-nondeficient group (23.6 ± 2.7 versus 37.3 ± 3.8 arbitrary units, P = 0.004). No significant differences were observed between the two groups in their hsCRP levels, mean age, estimated glomerular filtration rate, systolic blood pressure (SBP) and diastolic blood pressure (DBP) and glycosylated haemoglobin. CONCLUSIONS Microvascular endothelial function was significantly reduced in diabetic nephropathy patients with deficient vitamin D levels compared with those with nondeficient levels.