We reviewed our experience with 51 renal transplants to evaluate the accuracy of antegrade pyelography as compared with that of sonography and nuclear renography in the diagnosis of transplant obstruction. Also, the results of percutaneous treatment were analyzed in 44 of these patients. Obstruction was clinically suspected in all of the patients (increased serum creatinine levels and decreased urine output). Antegrade pyelography showed obstruction in 44 (86%) of the 51 patients, and nephrostomy drainage catheters were inserted. Sonography showed pyelocaliectasis in all 49 cases in which it was performed; in 42, the pyelocaliectasis was due to obstruction (14% false-positive rate). Nuclear renography showed obstruction in only six (18%) of 33 cases in which it was performed; all six cases proved to be obstructed (0% false-positive rate and 82% false-negative rate). Twenty-two (50%) of the 44 patients treated with nephrostomy drainage were managed successfully without surgical intervention; seven of these 22 required balloon dilatation or ureteric strictures in addition to catheter decompression of the collecting system. The average duration of catheterization for successful percutaneous treatment was 35 days. This experience suggests that antegrade pyelography has a definite role in the workup of patients suspected of having renal transplant obstruction. The percutaneous access permits successful catheter drainage. Compared with antegrade pyelography, sonography is reasonably accurate in determining the presence of urinary obstruction, although false-positive diagnoses are found in a substantial number of patients. Nuclear renography is not, however, a useful indicator of obstruction owing to its high false-negative rate. Percutaneous treatment of urinary obstruction in transplantation patients proves successful in approximately 50% of cases.