Low levels of magnesium have frequently been reported in diabetes mellitus especially in poorly controlled Type 1 (insulin-dependent) diabetic patients. Furthermore hypomagnesaemia might contribute to insulin resistance in Type 2 (non-insulin-dependent) diabetes. As the influence of improved metabolic control on plasma magnesium levels is unknown in Type 2 diabetic patients we studied magnesium plasma levels in 50 patients 1) before, 2) one and 3) three months after the initiation of insulin therapy or intensified treatment with oral hypoglycaemic agents. Magnesium plasma levels were measured by a colorimetric method and were significantly reduced in diabetic patients compared to healthy control subjects (0.79 +/- 0.01 mmol/l vs 0.88 +/- 0.01 mmol/l; p less than 0.0001). Metabolic control was significantly improved as documented by reduced HbA1C levels in both insulin-treated patients or the patients on oral hypoglycaemic agents (p less than 0.003). However, plasma magnesium levels remained unchanged during the follow-up in the insulin-treated group (1: 0.79 +/- 0.02 mmol/l; 2: 0.81 +/- 0.02 mmol/l; 3: 0.79 +/- 0.01 mmol/l) as well as in the patients on oral hypoglycaemic agents (1: 0.79 +/- 0.03 mmol/l; 2: 0.78 +/- 0.02 mmol/l; 3: 0.84 +/- 0.04 mmol/l). This study shows that even marked improvement of glycaemic control does not correct hypomagnesaemia in Type 2 diabetes. We conclude that hypomagnesaemia might be related to the insulin-resistant state and that possible beneficial effect of chronic magnesium administration should be evaluated in these patients.