Summary— One hundred and thirty-one patients operated on for pyonephrosis within a 4-year period are reviewed. Male to female ratio was 1∶2 and urolithiasis was the most common aetiological factor (70.2%). Primary nephrectomy proved to be a very satisfactory mode of treatment. Preliminary nephrostomy no longer appears necessary other than in exceptional circumstances. In a selected group of patients with pyonephrosis following acute obstruction of the kidney by a stone, successful management was achieved by prompt relief of the obstruction.