The aim of this study was to assess the intra-individual reproducibility of ⁹⁹Tc^m-mercaptoacetyltriglycine (⁹⁹Tc^m-MAG3) clearance in routine clinical practice in various circumstances using a single-sample method. Three groups of 30 patients each were enrolled in the study. Investigations were repeated on the same day with two different levels of activity (group A), within 1 week (group B), or after 1 year (group C). Blood samples were taken after 20 and 25 min and clearance values were averaged to minimize incidental errors after calculation according to the formula proposed by Bubeck et al. For clearance estimation on the same day, time-dependent background correction for residual activity was carried out. The mean difference was -2.0%, +6.0% and -1.5% (S.D. = 6.3%, 15.7% and 11.7%) for groups A, B and C respectively, taking into account values >100 ml min^-1 only. We demonstrated that ⁹⁹Tc^m-MAG3 clearance is highly reproducible under the same conditions. However, a repeat clearance measurement on a separate day revealed large differences. Therefore, in follow-up studies for evaluation of changing kidney function (e.g. surgery or chemotherapy), differences should exceed 30% (i.e. 2 S.D. of group B) to be statistically significant.