Magnesium concentrations in plasma, erythrocyte and platelet, and plasma and urine levels of the soluble form of Intercellular Adhesion Molecule-1 (sICAM-1) were evaluated in subjects with insulin dependent diabetes mellitus (IDDM) with or without microalbuminuria, and in a control group of healthy subjects. Using a recently introduced technique, we found that magnesium concentrations in platelets in diabetic subjects with microalbuminuria were lower than in diabetics with normal albuminuria (1.859 +/- 0.47 vs 2.065 +/- 0.62 mumol/10(8) cells; P < 0.05). Moreover, IDDM subjects had higher plasma sICAM-1 levels than control subjects; no difference, however, was found between sICAM-1 concentrations in the two groups of diabetics. An inverse correlation was found between intraplatelet magnesium and plasma sICAM-1 levels (r = - 0.64; P < 0.05) in the diabetics with microalbuminuria. It is concluded that the reduced intraplatelet magnesium content may contribute to the progression of the vascular complications in IDDM subjects with microalbuminuria.