This article evaluates the clinical usefulness of Tc-99m-ethylenedicysteine (EC) in patients with various renal disorders. In addition, extraction ratios of Tc-99m-EC in five volunteers Were also determined. Methods: Twenty patients were intravenously injected with 200 MBq Tc-99m-EC and 2.5 MBq [(131)l]orthoiodohippurate (OIH) simultaneously and 11 blood samples were withdrawn within 60 min. Plasma clearance was determined on the basis of a two-compartment model. Imaging was performed in the posterior projection by acquiring three sets of images. Extraction ratios were determined from the blood samples obtained from the renal vein and abdominal aorta Results: Renal clearance of Tc-99m-EC was significantly lower than that of OIH (p = 0.0003) with good correlation (r = 0.93). Volume distributions of Tc-99m-EC and OIH were 26584 +/- 10807 ml/1.73 m(2) and 23148 +/- 7602 ml/1.73 m(2), respectively (p = 0.047). The clearance half-lives of Tc-99m-EC and OIH were 98 +/- 54 min and 74 +/- 54 min, respectively (p = 0.049). Protein binding of Tc-99m-EC (33 +/- 3.2% was significantly less than that of OIH (62 +/- 2.8%) (p < 0.0001). Red blood cell binding of Tc-99m-EC was almost negligible (5.7 +/- 4.3%). Similar extraction ratios were obtained from blood (0.68 +/- 0.08) and plasma (0.70 +/- 0.07) (p = 0.062). The 60-min excretion fractions were similar for Tc-99m-EC and OIH, with Values of 50% +/- 20% and 51% +/- 19%, respectively (p = 0.9). Conclusion: Technetium-99m-EC is a suitable radio-pharmaceutical for routine renal dynamic studies. Although the biological behavior of Tc-99m-EC seems to be different from that of OIH, their clearances demonstrate high correlation. Technetium-99m-EC provides excellent quality images and has high potential in the evaluation of quantitative renal functions.