Forty patients with acute onset renal colic were studied using technetium-99m DTPA renal scintigraphy. Thirty-two were found to have varying degrees of obstruction. In the obstructed group the level was determined by the scan alone in 24 of 32 cases (75%). When the analysis included an abdominal radiograph, the level of obstruction was ascertained in 29 of 32 cases (91%). A delayed nephrogram, as evidenced by an "empty renal pelvis sign" at 5 minutes, was observed in all but one of the cases of obstruction (97%). Delayed clearance of activity from the pyelocalyceal system or ureter was also seen in 97%. In all cases of obstruction either one or both of these signs were observed. However, delayed clearance was found to be a nonspecific finding and was noted in 25% of nonobstructed cases. The finding of the "empty renal pelvis sign" at 5 minutes was found to be specific and was not seen in the nonobstructed group. The authors conclude that radionuclide renal scanning is an effective method for evaluating selected patients with suspected renal colic.