The relation between the timing of prophylactic antibiotic administration and the occurrence of bacteraemia during transurethral operations was studied in 112 patients whose urine was infected before operation. Blood cultures taken during operation were positive in 15 (60%) of 25 patients who did not receive appropriate antibiotics, in 13 (21%) of 63 patients who were given appropriate antibiotics less than 24 h before operation, and in none of 24 patients in whom antibiotic "cover" was started more than 24 h before operation. In all cases the bacteraemia was transient. No patient developed septicaemia. The implications of these findings for the optimum timing of antibiotic administration to patients with preoperative bacteriuria are discussed.