The incidence of urinary infection and its source after transurethral resection of bladder tumors were investigated. Patients undergoing transurethral prostatectomy constituted the control group. All patients had proved preoperative sterile urine and did not receive any antimicrobial treatment. The incidence of urinary infection following transurethral resection of bladder tumors was 19.4 per cent, with an incidence of positive tumor chip cultures of 3.2 per cent. The incidence of urinary infection after transurethral prostatectomy was 32 per cent, with an incidence of positive prostate chip cultures of 8 per cent. We concluded that the source of urinary infection following transurethral resection of bladder tumors and transurethral prostatectomy in most cases is not from within the tissue but rather from perioperative manipulations.