To determine the accuracy of dimercaptosuccinic acid (DMSA) scintigraphy in assessing the differential function between upper and lower moieties of a refluxing duplex system. Between 1990 and 1996, 20 patients underwent lower moiety heminephroureterectomy. All patients had their differential renal function assessed by DMSA scintigraphy before and after surgery, the results being assessed by comparing the predicted and actual loss of renal function. The mean (SD, range) function attributed to the scarred lower moiety (predicted loss of function) was 4.3 (4.9, 0-14)%, which compared with an actual mean loss of function of 6.8 (5.1, +4 to -16)%. Thus the mean (SD, range) difference between the predicted and actual loss of renal function was -2.1 (3.2, +4 to -8)%. DMSA scintigraphy provides an accurate assessment of the differential renal function between upper and lower moieties in a duplex system. The decision to proceed to lower moiety heminephroureterectomy can be logically based on the information gained from DMSA scintigraphy.