It has been shown that technetium-99m ethylenedicysteine (Tc-99m-EC) clearance displays a strong correlation with orthoiodohippurate clearance and that it is possible to estimate effective renal plasma flow from Tc-99m-EC clearance. However, in routine practice, when monitoring renal function of patients by clearance determinations it is imperative to test the reproducibility of the clearance technique in order to decide whether the changes in renal function are related to disease or not. The aim of this study was to rest the reproducibility of Tc-99m-EC clearance. The study group comprised 13 patients aged between 24 and 58 years (eight female and five male). Two patients had single kidneys due to agenesis and hypoplasia. Plasma creatinine and BUN levels were within the normal range and remained stable between two clearance studies. None of the patients were receiving medication. Two Separate multiple sample clearance studies were performed in each patient within a week. The difference between two studies was expressed as a percentage of the mean value of the two studies, and the standard deviation of these percentages represented the precision. The plasma clearance of Tc-99m-EC in the first-step studies ranged from 190 ml/min to 561 ml/min with a mean of 411+/-101 ml/min. The clearances obtained from the second-step studies ranged from 248 ml/min to 552 ml/min, with a mean of 387+/-94 ml/min. There was no statistically significant difference between the two clearance studies (P>0.05). There was a mean difference of 5.3% between the first and second clearance studies, and the precision was found to be 12.7%. In conclusion, this study suggested that consecutive Tc-99m-EC clearance measurements should be interpreted cautiously.