Extracellular fluid (ECF) is associated with blood pressure, but reports on the status of the ECF volume in hypertension have been inconsistent. The aim of this study was to assess the ECF volume status in hypertensives with regard to body size in a large cohort. We performed a single-center case-control observation study for patients who visited the outpatient hypertension clinic and health examination center. For all eligible participants, we examined the body composition, including fluid compartments, using a noninvasive bioimpedance analysis. Of 2934 subjects (women 1365, 57.5 ± 12.2 years), 1166 subjects were normotensive and 1768 subjects were hypertensive. The ECF volume was larger in female hypertensives than in normotensives but was not different between the male groups. However, the relative ECF, defined as the ratio of ECF to body mass index (BMI), was significantly lower in hypertensives of both genders (P<0.001). ECF revealed an almost twofold stronger correlation with the fat-free mass (FFM) (r=0.9 in both genders) than with the fat mass, BMI or waist circumference and a negative correlation with age. In contrast, the relative ECF was positively correlated only with the FFM and inversely correlated with the other factors. In the multivariate logistic regression analysis, an increase of 1 s.d. in the relative ECF decreased the relative risk of hypertension by 30% in women (odds ratio (OR), 0.70; 95% confidence interval (CI), 0.56-0.87) and by 28% in men (OR, 0.72; 95% CI, 0.60-0.86), but the ECF was not independently associated with hypertension in either gender.The ECF with regard to the body size was contracted in hypertensives and independently associated with hypertension, whereas the absolute ECF volume was not.