A series of 70 pyonephrotic kidneys drained by percutaneous nephrostomy tube was examined to evaluate the contribution of radiologic imaging to the diagnosis of pyonephrosis and to assess the diagnostic and therapeutic role of drainage by percutaneous nephrostomy catheter. The diagnosis of pyonephrosis is suspected when the clinical symptoms of fever and flank pain are combined with the radiologic evidence of obstruction to the urinary tract. Sonography gives a prompt diagnosis of hydronephrosis, and needle puncture of the kidney yields pus and establishes the presence of pyonephrosis. A percutaneous nephrostomy catheter is then inserted and serves for initial drainage of infected urine and for evaluation of residual kidney function before definitive surgery. The nephrostomy catheter is used for diagnostic nephrostograms, ureteral perfusions, therapeutic dissolution of stones, and indefinite drainage of the kidney. In 10 azotemic patients, the blood urea nitrogen and serum creatinine values returned to normal levels after antibiotic therapy and nephrostomy drainage of infection. Long-term evaluation of residual renal function by means of an excretory urogram or a renogram was available in another 26 patients and 25 of them showed function of the previously pyonephrotic kidney.