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The ability to objectively determine the degree of tissue edema and to monitor on-line fluid balance in critically ill patients would be a clinical benefit. In this prospective descriptive trial, we evaluated a new noninvasive method--dielectric constant of skin and subcutaneous fat (SSF)--in assessing fluid balance during cardiac surgery. The dielectric constant at the applied high radiofrequency is a direct measure of tissue water content. Twenty-nine patients with elective cardiac surgery participated in the study. Dielectric constants on forearm, thigh and abdomen were measured before surgery, within 1 h after surgery and in the first, second, third and fourth postoperative morning. At the same time the patients were weighed, except immediately after the operation and the first postoperative day when fluid balances were calculated. A statistically significant correlation (r = 0.60, p < 0.01) was found between the increase of the dielectric constant of SSF and weight gain of the patients from the baseline to the second postoperative morning. From the second to the fourth postoperative day when the patients were losing the weight, a statistical significant correlation between the dielectric constant and weight loss was not found. The results suggest that the measurement of the dielectric constant is a promising new method in assessing the fluid status of operated patients during the time the patients cannot be weighed.