There is no definite way to estimate the ideal body fluid level in uremic patients. We developed a simple method to quantitatively estimate the ideal body weight. Hemofiltration (HF) was performed 9 times in 5 uremic patients. Urea-nitrogen concentration was measured in plasma before and after HF and in total ultrafiltrate mixture. Creatinine concentration was measured in plasma after HF and before the next HF and in urine collected between those two HF therapies. Total body fluid volume, TBF, was obtained, based on urea kinetics principles, as 36.8 +/- 1.8 L after HF assuming that urea is distributed uniformly within total body fluid. Creatinine generation rate, Gcr, was calculated as the sum of urinary creatinine excretion rate, creatinine accumulation rate within body fluids and extra-renal creatinine excretion rate. It is known that solid mass, SM, is related to Gcr and that the TBF to lean body mass (= SM + TBF) ratio is constant under ideal fluid level conditions. Thus, TBF under ideal body fluid level conditions (37.8 +/- 3.0 L), SM (13.7 +/- 1.1 kg) and fat mass (3.7 +/- 2.3 kg) were derived, and finally we found body weight value under ideal fluid level conditions to be 55.1 +/- 4.2 kg. The ideal body weight estimated this way was found to be significantly correlated with the dry weight (54.0 +/- 3.5 kg) determined clinically by conventional means (r = 0.9; p < 0.01); no significant difference was found between these body weight values. Our results suggest that the ideal body weight can be estimated simply by applying both urea and creatinine kinetics principles in uremic patients.