Patients treated by transurethral operation were studied prospectively to determine the risk of post-operative septicaemia and whether it could be prevented by chemotherapy. Septicaemia was provoked by operation or catheter removal only in patients with infected urine. Septicaemia followed three of 206 operations in patients with initially sterile urine, which had become infected before the catheter was removed. Eleven of 179 (6.15%) operations in patients with infected urine not receiving appropriate antibiotics were followed by septicaemia. There was no septicaemia after 180 operations in similarly infected patients who received appropriate antibiotics from 2–12 h before operation. Septicaemia in patients with infected urine can be prevented by chemotherapy starting soon before operation, using appropriate antibiotics selected by rapid urine culture with sensitivity tests at the time of admission to hospital.