We assessed blood volume (BV) in 20 edematous patients with nephrotic syndrome and in 32 subjects without renal or other disease liable to induce BV variation. Two methods were chosen, one using 131I-albumin and the other 51Cr red blood cells. Among the 20 patients, 11 presented minimal-change lesions, and 9 had histological glomerular impairment. The BV was significantly higher when measured with 131I-albumin in both nephrotic patients and in controls. However, in patients with nephrotic syndrome, the values obtained from the measurement of BV by 131I-albumin showed an increase of only 1%. Comparison of BV values of nephrotic patients and controls showed that BV is equal or higher in two thirds and lower in one third, respectively. The same incidence of histological glomerular lesions was observed in both groups. In conclusion, this study demonstrates that the method using 131I-albumin to measure BV in nephrotic syndrome is reliable. BV is decreased in one third of the patients and is not related to the presence or absence of histological glomerular lesions.