The indication of low salt diet for the management of hypertension associated with pregnancy is controversial. We studied the effect of a low-salt diet (less than 5g/day) on pregnancy-induced hypertension compared to patients with hypertension due to chronic renal failure and essential hypertension. In chronic renal failure, mean blood pressure decreased from 115.3 +/- 3.0mmHg to 92.1 +/- 2.6mmHg (p < 0.001) and in essential hypertension, from 117.7 +/- 3.1mmHg to 108.5 +/- 3.5mmHg (p < 0.01). However, in pregnancy-induced hypertension, the blood pressure did not change significantly. CUA/Ccr ratio, the indicator of plasma volume, decreased significantly from 8.7 +/- 1.5% to 3.8 +/- 0.7% (p < 0.001) after salt restriction. CUA and mean blood pressure in patients with preeclampsia were negatively correlated significantly (r = -0.51, p < 0.05). There was a significant correlation between CUA and urinary sodium excretion (r = 0.67, p < 0.001). These results indicate that a low-salt diet is not only ineffective, but also accelerates volume depletion in preeclampsia.