We report the use of cefoperazone in 62 cases of serious infection, most of which occurred in patients with renal impairment. 43 severe or complicated urinary tract infections, 11 cases of pneumonia and 8 with other severe sepsis were treated with cefoperazone 1 to 2 g twice daily usually for 5 to 10 days. Of the patients with urinary tract infection, all who were symptomatic showed a rapid clinical response; 26 (61%) were cured including 11 of 16 with chronic renal failure; 12 relapsed and 5 were reinfected with a different pathogen. All of these patients were infected by organisms sensitive to cefoperazone by disc testing but in 5 of those who relapsed the cefoperazone MIC was in fact greater than or equal to 50 microgram/ml. Ten of 11 cases with radiologically confirmed pneumonia were cured with cefoperazone. 7 episodes of pneumonia were in patients with end-stage chronic renal failure (6 were on dialysis) and 1 was in a patient with acute renal failure. Seven of 8 cases with severe sepsis were cured with cefoperazone. 1 patient was withdrawn from the study when acute bronchospasm followed a 2 g intravenous dose. 2 of the successfully treated patients had functioning renal transplants, 2 of 3 with severe chronic renal failure were on dialysis and 1 had acute renal failure. Side effects included minor disturbances of liver function in 6 patients (11%), diarrhoea in 7 (13%) and marked alcohol intolerance in one, 4 patients with chronic renal failure developed a coagulation disorder which was corrected with vitamin K. None of the patients showed deterioration in renal function while receiving cefoperazone. Cefoperazone promises to be an effective drug for the treatment of a wide spectrum of severe infections in hospitalised patients including those with impaired renal function.