When imaging renal transplants with tubular agents, such as mercaptoacetyltriglycine, marked parenchymal retention is a hallmark of acute rejection (AR) or acute tubular necrosis (ATN). (AR can be distinguished from ATN by the time course on serial studies.) The quantitative relationship of retention to uptake can be measured by dividing the background-corrected renal activity at 20 min by that at 3 min. Methods: The diagnostic value of this ratio (R(20/3)) was tested in a series of 555 renograms. Because patients with mild disease have minimal abnormalities, the patients were ranked by their estimated severity of disease (1-4 for abnormal and 0 for normal). Results: R(20/3) was found to correlate strongly with severity of ATN (Spearman's ρ = 0.879, p < 0.001, n = 168) and also with severity of AR (ρ = 0.888, p < 0.001, n = 267). There were two (3%) false-positive results in 64 normal patients. Conclusion: If 0.8 is taken as the upper limit of normal for R(20/3), then among patients with disease severity 3 or 4, there were no false-negative findings in 104 patients with ATN or in 203 patients with AR. R(20/3), despite its simplicity, is an effective diagnostic parameter.