Serum and erythrocyte magnesium concentrations (S-Mg, E-Mg) were measured in 380 Japanese junior high school students, and the relationship to blood pressure and a family history of hypertension were studied. Systolic blood pressure was higher in the subjects with a positive family history of hypertension [FH(+)] than in those with a negative family history [FH(-)], whereas E-Mg was lower in the FH(+) group than in the FH(-) group with a significant different in boys. Furthermore, in the FH(+) group, systolic blood pressure was significantly higher in the subjects with lower S-Mg and E-Mg than in those with higher S-Mg and E-Mg. In the FH(-) group, however, no difference was observed in blood pressure levels between the two subgroups. These findings suggest that magnesium deficiency is partially responsible for a rise of blood pressure in the FH(+) children, and that a genetic predisposition of hypertension may be closely related to magnesium metabolism.