The plasma concentration of ionized magnesium (iMg++), the serum total magnesium concentration (tMg), and lipoprotein status were assessed in 29 elderly men with impaired insulin sensitivity but who were otherwise healthy and not receiving antidiabetic pharmacologic treatment. Plasma iMg++ was inversely correlated with serum concentrations of VLDL cholesterol (r = -0.39, P < 0.04), LDL cholesterol (r = -0.57, P < 0.002), total cholesterol (r = -0.55, P < 0.002), VLDL triacylglycerol (r = -0.36, P < 0.05), LDL triacylglycerol (r = -0.48, P < 0.01), and total triacylglycerol (r = -0.41, P < 0.03). The serum total magnesium concentration also tended to be inversely correlated with these lipid fractions, but not significantly so. Furthermore, iMg++ was inversely correlated with the apolipoprotein B concentration (r = -0.56, P < 0.002) and with the ratio of apolipoprotein B to A-I in serum (r = -0.58, P < 0.001). The corresponding values for tMg were r = -0.37 (P = 0.05) and r = -0.37 (P = 0.05), respectively. Insulin increment and the area under the insulin curve during an intravenous glucose tolerance test were inversely correlated with iMg++ [r = -0.56 (P < 0.01) and r = -0.64 (P < 0.0005), respectively], but were not significantly correlated with tMg. Insulin-mediated glucose uptake and the insulin sensitivity index during the hyperinsulinemic euglycemic clamp test were not significantly correlated with tMg (r = 0.05 and 0.12, respectively; NS) nor with iMg++ (r = 0.23 and 0.31, respectively; NS). Atherogenic lipid fractions and some glucometabolic variables were more closely correlated with iMg++ than with tMg.