We investigated the source of organisms detected in blood specimens obtained during and immediately following prostatic surgery under perioperative antibiotic use, as related to the results of cultures of preoperative urines and prostatic tissues. Ninety patients with benign prostatic hyperplasia were studied. The incidence of bacteremia was 28.9%: 53.7% in patients with preoperative bacteriuria and 8.2% in those without bacteriuria, a significant difference (p < 0.01). Of these bacteremic patients, 5, who had preoperative bacteriuria, developed septicemia. Bacteremia developed more frequently in patients with positive than in those with negative prostatic cultures; this difference was significant (p < 0.05). The species in 80.8% of the isolates from blood specimens were identical with those isolated from preoperative urines, and the species in 53.8% were identical with those from prostatic tissues. These results indicate that in patients with urogenital infection, especially bacteriuria, septicemia can result from prostatic surgery even under perioperative antibiotic use. The incidence of postoperative bacteriuria was approximately 20% in the preoperative nonbacteriuric patients regardless of the duration of chemo-prophylaxis.