In newly diagnosed insulin-dependent diabetes mellitus, the mechanisms underlying the concomitant occurrence of magnesium deficiency and normal blood magnesium concentration are unknown. The renal handling of magnesium was, therefore, studied in 37 children with newly diagnosed insulin-dependent diabetes mellitus and in 13 controls. Circulating magnesium levels were similar in patients and controls (0.86 vs. 0.84 mmol/l). However, the urinary excretion of magnesium was significantly higher in patients (90.6 vs. 32.2 mumol/l GFR). In the patients a significant positive correlation was found between excretion of magnesium and glycosuria or blood hydrogen ion activity. It is concluded that osmotic diuresis and acidosis increase magnesium excretion in newly diagnosed diabetes mellitus.