This study was undertaken to compare image quality, renal handling and plasma clearance of 99Tcm-mercaptoacetyltriglycine (99Tcm-MAG3) with those of 123I-orthoiodohippurate (123I-OIH). 99Tcm-MAG3 was prepared in-house using an instant kit vial and 123I-OIH was commercially available in a form already labelled. Both studies were undertaken in 20 patients with nephrourological disorders after intravenous (i.v.) injection of 300 MBq 99Tcm-MAG3 and 34 MBq 125I-OIH within the time interval of 3 to 4 days. 99Tcm-MAG3 was much superior to 123I-OIH in visualization of renal perfusion, renal parenchyma and urinary tract. The time parameters of the renogram, Tmax and T1/2 were significantly prolonged in 99Tcm-MAG3 compared to those for 123I-OIH (P < 0.05). Plasma clearance of 99Tcm-MAG3 was also slower than that of 123I-OIH but correlated well with that of 123I-OIH (r = 0.958, P < 0.01). The % renal uptake of 99Tcm-MAG3 per injected dose, which was calculated by the gamma camera method, also correlated well with effective renal plasma flow (ERPF) calculated by the single blood sampling method (r = 0.940, P < 0.01). The renal handling and plasma clearance of 99Tcm-MAG3 are not completely identical with those of 123I-OIH. However, 99Tcm-MAG3 has the clinical advantages of superior image quality, the feasibility of gamma camera measurement of quantitative renal uptake, and ready availability for routine clinical use.