Recent literature has questioned whether 99mTc- mercaptoacetyltriglycine (MAG3) clearance measurements are reproducible enough for routine clinical monitoring of renal function. For many years, we have routinely followed the renal function of patients with spinal cord injuries using a combination of radionuclide imaging and clearance measurement. Methods: In this study, we retrospectively review 1626 effective renal plasma flow (ERPF) measurements in 197 patients with paraplegia or quadriplegia performed over a 21-y period, using 131I-orthoiodohippurate (OIH) through 1990 and MAG3 since 1991. MAG3 clearance was divided by 0.53 to convert it to ERPF. Reproducibility was measured as pooled SD from the single-patient linear regression lines of ERPF versus time. Results and Conclusion: There was no significant difference between MAG3 (SD = 46 mL/min, n = 907) and OIH (SD = 52 mL/min, n = 719). The data were therefore combined to obtain the SD for a single ERPF measurement, which was 49 mL/min. The corresponding coefficient of variation was 8.5% of the mean value of 581 mL/min. In our experience, this is adequate for monitoring the renal function of these patients.