This study was performed to elucidate the effect of a hypocaloric diet on blood pressure and sodium balance in overweight women with essential hypertension. Hospitalized patients were fed a hypocaloric diet (900 kcal/day) for 3 weeks after being fed a standard diet (SD) for more than 2 weeks. During the study, the sodium and potassium intake was kept constant as was the level of exercise activity. After the mean arterial pressure (MAP) and body weight had stabilized during the last week on the SD, caloric restriction significantly reduced the body weight by 1.5kg in the first week and by 0.8-0.9 kg in subsequent weeks, and reduced the MAP by 5mmHg and by 1-3mmHg, respectively. Although caloric restriction reduced the excretion of urinary norepinephrine and C-peptide, and elevated the serum levels of ketone bodies that would induce natriuresis, the urinary sodium excretion remained unchanged, indicating that sodium balance was unchanged even by caloric restriction. These results suggest that caloric restriction lowers the blood pressure in a short time with no significant change in sodium balance. (Hypertens Res 1992; 15: 137-142)