Diffusion of ofloxacin (OFLX) into prostatic tissue was studied in 31 patients with benign prostatic hypertrophy. Concentrations of OFLX in the serum and prostatic tissue were measured at scheduled intervals after 200 mg OFLX oral administration. The mean OFLX level in prostatic tissue and tissue/serum ratio at 2 hours, 4 hours and 6 hours was 3.33 +/- 0.96 micrograms/g (1.25 +/- 0.28) in 10 patients, 2.21 +/- 0.55 micrograms/g (0.92 +/- 0.32) in 9 patients and 2.10 +/- 0.99 micrograms/g (1.01 +/- 0.23) in 12 patients, respectively. OFLX levels in prostatic tissue covered the minimum inhibitory concentration for several pathogenic bacteria detected from the infected prostatic fluid. Therefore, OFLX was thought to be a very useful drug for the treatment of bacterial prostatitis and postoperative infection of prostatic surgery.