A double-blind, randomised, placebo-controlled study was carried out to determine the incidence and significance of bacteriuria in 110 patients undergoing transurethral resection of the prostate (TURP) and to assess the effect of a single pre-operative dose of Ciprofloxacin, a 4-quinolone antibiotic. Fifteen (68%) of the 22 patients in the placebo group with a positive post-operative urine culture subsequently developed a clinically apparent urinary tract infection (UTI) or received antibiotics in view of a positive urine culture. Adequate prostatic concentrations of Ciprofloxacin were achieved in all who received the drug. A significant reduction in the number of positive post-operative urine cultures and urinary tract infections requiring antibiotic therapy was achieved in this group. Six patients (5.5%) developed clinical evidence of septicaemia, 5 of whom were in the placebo group. No organisms resistant to Ciprofloxacin were encountered. Prior to surgery, 19% of all patients were found to have previously unsuspected bacteriuria. Ciprofloxacin tended to reduce the chances of this group developing a UTI or requiring antibiotics. Further, there was a highly significant reduction in post-operative infective complications in those with sterile urine at the time of resection who had received the drug. This study suggests that antibiotic cover for TURP is of clinical benefit. Ciprofloxacin may prove suited to this purpose, although further experience with the drug is still required.