Twenty five patients with benign prostate hypertrophy were administered ofloxacin (OFLX) simultaneously with norfloxacin (NFLX) in 6 patients, ciprofloxacin (CPFX) in 10, tosfloxacin (TFLX) in 5 and enoxacin (ENX) in 4 patients. The dose of these new quinolones was 100 mg (except 150 mg of TFLX). Their concentrations in the serum and the prostate tissue were measured by high performance liquid chromatography (HPLC). The serum concentration of OFLX was significantly higher than that of NFLX, CPFX, TFLX or ENX. The prostate tissue concentration of OFLX was significantly higher than that of CPFX or TFLX. The ratio of tissue versus serum concentration of each new quinolone agent was not significantly different. The high OFLX tissue concentration appeared to be caused by the high serum concentration.