A gamma camera method with no blood sampling assesses differential renal function based on initial renal uptake, and requires background correction. The effect of background correction on estimates of initial renal uptake was compared on renograms using Tc-99m MAG3 and Tc-99m DTPA renographies in 14 patients. Renal counts for 2-3 minutes were obtained using three sets of regions of interest (ROIs): rectangular renal and subrenal background ROIs, rectangular renal and perirenal background ROIs, and hand-drawn renal and subrenal background ROIs. Correlations between estimates of initial renal uptake by the three methods were higher for Tc-99m MAG3 renography than for Tc-99m DTPA imaging, suggesting higher reproducibility in evaluating differential renal function using Tc-99m MAG3 renography and a gamma camera method. Although the ratio of counts in the suprarenal area to those in the subrenal area was significantly higher for the right side in the Tc-99m MAG3 study, probably because of hepatic activity, the relative uptake in the right kidney for Tc-99m MAG3 did not differ from that for Tc-99m DTPA. Initial uptake of Tc-99m MAG3 was well correlated with, and about threefold that, of Tc-99m DTPA. It was suggested that initial renal uptake of Tc-99m MAG3 may be an alternative to that of Tc-99m DTPA, with less dependence on background correction in evaluating differential renal function.