In 88 patients with the nephrotic syndrome (NS) we estimated plasma volume (PV) with 131I-albumin (RISA) and calculated blood volume (BV) from PV and whole body hematocrit (Ht). To assess whether this method gives erroneously high values in the NS, the transcapillary escape rate of albumin (TER) and large vessel Ht/whole body Ht ratio (F-cell ratio) were measured in two subsets of these patients (n = 19 and 24, respectively). Although TER appeared to be increased and F-cell ratio slightly decreased in the NS as compared to normals, it could be calculated that overestimation of PV and BV from these sources will not exceed ± 1%. Taking these errors into account PV and BV appeared normal or increased in the NS (62.8 ± 9.6 and 94.9 ± 15.1 ml/kg vs. 56.0 ± 7.1 and 88.5 ± 10.1 ml/kg lean body mass in 51 normal controls). We conclude that PV can be measured reliably in the NS with 131I-albumin, and that even after appropriate corrections are made, PV and BV are normal or increased in most patients with th