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BACKGROUND: Low vitamin D levels can be associated with albuminuria, and vitamin D analogs are effective anti-proteinuric agents. The aim of this study was to investigate differences in vitamin D levels between those with micro- and those with macroalbuminuria, and to determine whether low dose cholecalciferol increases vitamin D levels and ameliorates albuminuria. METHODS: Two studies were performed in which 25-OH vitamin D(3) (25(OH)D(3)) concentrations were determined by electrochemiluminescence immunoassay: 1) a cross-sectional study of patients with type 2 diabetes mellitus (T2DM) (n = 481) and healthy controls (n = 78); and 2) a longitudinal study of T2DM patients with albuminuria treated with conventional doses, 800 IU, of cholecalciferol for 6 months (n = 22), and a control group (n = 24). RESULTS: 1) Cross-sectional study: Compared to controls and T2DM patients with normoalbuminuria, serum 25(OH)D(3) concentrations were significantly lower in patients with macro-albuminuria, but not in those with micro-albuminuria. Serum 25(OH)D(3) levels were independently correlated with microalbuminuria. 2) Longitudinal study: Cholecalciferol significantly decreased microalbuminuria in the early stages of treatment, in conjunction with an increase in serum 25(OH)D(3) levels. CONCLUSIONS: Low vitamin D levels are common in type 2 diabetic patients with albuminuria, particularly in patients with macroalbuminuria, but not in those with microalbuminuria. Conventional doses of cholecalciferol may have antiproteinuric effects on Chinese type 2 diabetic patients with nephropathy.