We evaluated plasma, erythrocyte and platelet magnesium levels in patients with insulin-dependent diabetes mellitus (IDDM) with normoalbuminuria (N = 10), microalbuminuria (N = 10), and clinical proteinuria (N = 7), and in a group of healthy subjects (N = 10). We found that IDDM patients had lower platelet magnesium levels when compared to controls. Lower platelet magnesium concentrations were found in patients with microalbuminuria (1.859 +/- 0.47 vs 2.340 +/- 0.46 mumol/10(8) cells, p < 0.05) and in those with clinical proteinuria (1.522 +/- 0.19 vs 2.340 +/- 0.46 mumol/10(8) cells, p < 0.01) with respect to the group with normoalbuminuria. In the groups with microalbuminuria and clinical proteinuria we detected a negative correlation between HbA1c and both plasma and platelet magnesium. Our findings indicate that microalbuminuria and clinical proteinuria are associated with an altered magnesium homeostasis. In particular, decreased platelet magnesium concentrations could represent an additional risk factor in the pathogenesis of microvascular complications of diabetes.