Interrelations between magnesium and glucose metabolism were studied in 215 insulin-treated diabetic out-patients aged 7-70 years. All had normal serum creatinine concentrations (below 115 mumol/l) and none had other diseases or received drugs known to interfere with mineral metabolism. A definite hypomagnesaemia (less than normal mean - 2 SD) and hypermagnesiuria (greater than normal mean + 2 SD) occurred in 38.6% and 55% of the patients. In the presence of hypermagnesiuria the serum magnesium concentration was inversely correlated to the urinary magnesium excretion rate (R = -0.23, P less than 0.02). Serum magnesium correlated inversely with both fasting blood glucose (R = -0.32, P less than 0.001) and the urinary glucose excretion rate (R = -0.22, P less than 0.005). The urinary magnesium excretion rate correlated directly with the same variables (R = 0.27, P less than 0.001 and R = 0.58, P less than 0.001, respectively). These data indicate that the net tubular reabsorption of magnesium is decreased in diabetic patients in presence of hyperglycaemia, leading to hypermagnesiuria and hypomagnesaemia.