The aim of this study was to evaluate relationship between serum magnesium and course of diabetes mellitus and also to find out, if there is any relation between serum magnesium and various complications of diabetes mellitus. A cross-sectional study was conducted to examine the relationship between serum magnesium in 50 type 1 and type 2 diabetic patients with or without complications and 40 normal healthy persons. Serum magneisum estimation was done using calmagite dye method using autoanalyser (Beckman DU clin systems). Serum magnesium levels in diabetic population was significantly low (1.93 +/- 0.282 meq/l) in comparison to control (2.25 +/- 0.429 meq/l). It was statistically significant (+3.84; p < 0.005). Serum magnesium was significantly low in diabetes with complication than without complications (p < 0.001). Duration of diabetes and serum magnesium were inversely related. Poor glycaemic control was associated with hypomagnesaemia (-2.623; p < 0.05). There was strong association between hypomagnesaemia and retinopathy (1.76 +/- 0.26), obesity (1.878 +/- 0.326) and hypertension (1.75 +/- 0.071) and it was statistically significantly (p < 0.005, 0.042, 0.000 respectively). Hence it is concluded that the change in serum magnesium level may have a bearing on the complication and morbidity in patients of diabetes mellitus.