Magnesium and potassium were analyzed in plasma, erythrocytes, and urine collected during 24 h and in muscle biopsies from 25 subjects with insulin-dependent, type I diabetes mellitus (IDDM). Magnesium was also measured in mononuclear cells. The results were compared with those of 28 healthy controls, and were also correlated with the degree of diabetic control as estimated by analysis of the level of glycosylated hemoglobin (HbA1c). Subjects with IDDM had significantly lower muscle (P < 0.01) and plasma (P < 0.001) concentrations of magnesium compared with those of healthy controls. The HbA1c levels correlated significantly with the concentrations of magnesium in muscle (r = −0.62, P < 0.001), plasma (r = −0.62, P < 0.001), and mononuclear cells (r = −0.47, P < 0.05). The results indicate that some patients with IDDM have lowered contents of magnesium in striated muscular and/ or plasma, and that those parameters are dependent on the degree of diabetic control.