The effective renal plasma flow (ERPF) of patients with renal-urinary disorders were estimated employing 99mTc-mercaptoacetyltriglycine (99mTc-MAG3) and a newly-developed clearance method which needs no blood sampling and is based on two-compartment fitting of the cardiac time-activity curves. In comparison with 123I-ortho-iodohippurate (123I-OIH) (n = 9), the disappearance rate constants and ERPF values estimated by the clearance method and the ERPF values by Schlegel's method showed a good correlation between the two agents (r = 0.88, r = 0.83 and r = 0.85, respectively). The ERPF values evaluated using 99mTc-MAG3 and the clearance method correlated well with those by Schlegel's method (n = 11, r = 0.95). These results suggest that 99mTc-MAG3 shows pharmacokinetics which are similar to those of 123I-OIH and that it might be a useful diagnostic tool for estimating the ERPF in patients with renal disorders.