Low vitamin D status increases the risk of death. Magnesium plays an essential role in vitamin D metabolism and low magnesium intake may predispose to vitamin D deficiency and potentiate the health problems. We investigated whether magnesium intake modifies the serum 25(OH)D3 concentration and its associations with mortality in middle-aged and older men. We included 1892 men aged 42-60 years without cardiovascular disease or cancer at baseline in 1984-1989 from the prospective, population-based Kuopio Ischaemic Heart Disease Risk Factor Study. Serum 25(OH)D3 was measured with the high-performance liquid chromatography using coulometric electrode array detection. Magnesium intake was assessed with 4-day food recording. Deaths were ascertained by a computer linkage to the national cause of death register. Deaths due accidents and suicides were excluded. Cox proportional hazards regression models were used to analyze the associations. The multivariate-adjusted hazard ratio (HR) for death in the lowest (49.4 nmol/L) serum 25(OH)D3 tertile was 1.31 (95 % CI 1.07-1.60, Ptrend = 0.01). Stratified by the magnesium intake, the higher risk was observed only in the lower magnesium intake median (