Serious morbidity from renal transplant biopsy is reported to be infrequent. However, 4 of 43 patients who had renal transplant biopsy between July, 1981, and March, 1984, experienced anuria from upper urinary tract obstruction by blood clots. Although these clots usually dissolve, 3 patients (7%) experienced persistent clot anuria and deterioration of renal function. Awareness of this complication is important. Retrograde pyelography and ureteral catheterization are preferred primarily for diagnosis and treatment. Percutaneous techniques are reserved for those cases in which the ureter cannot be catheterized cystoscopically.