The incidence of urinary infection before and after operation for benign prostatic hypertrophy has been reviewed in 140 patients. Repeated screening for bacteriuria was used throughout the hospital admission. Infection was present in 24% before operation and there was a significant correlation between the incidence of infection and the duration of preoperative catheterisation. Postoperative infection developed in 31% and there was no relation between this incidence and the type of prostatectomy; specific antibacterial therapy was significantly more effective in eradicating urinary infection acquired after operation than before. At the time of discharge from the ward, 10% had infected urine, but only 3% of patients had persistent infection at follow-up 6 weeks after operation.