The penetration of garenoxacin (GRNX) into serum, abscess, and palatine tonsil in patients with peritonsillar abscess was investigated and the pathogenesis of this disease and the effectiveness of GRNX were discussed. Ten patients with peritonsillar abscess who underwent abscess tonsillectomy were enrolled in the study. After oral administration with 400 mg of GRNX, the patients were put under general anesthesia and samples of blood, pus in the abscess, and both palatine tonsil were obtained 2 hours after the administration of GRNX. About 30 minutes later, those samples except for pus in the abscess were collected again. The samples were kept in -20°C until examination. The concentrations of GRNX in serum were determined by high-pressure liquid chromatography and that in pus and tonsil were determined by liquid chromatography-tandem mass spectrometry. Since one patient had abscess on both side and the concentration of GRNX in tonsil was higher than measuring limit in one patient, the data of GRNX concentration in those tonsils were excluded from the analysis. In results, the concentration of GRNX in serum, abscess, affected tonsil, and unaffected tonsil obtained in the first examination were 5.08 ± 1.54 μg/ml, 2.90 ± 2.27 μg/ml, 7.38 ± 3.74 μg/g, 8.34 ± 3.35 μg/g, respectively, and that in serum, affected tonsil, and unaffected tonsil obtained in the second examination were 4.60 ± 1.20 μg/ml, 6.74 ± 1.93 μg/g, 7.38 ± 2.70 μg/g, respectively. There were significant differences in the concentration of GRNX among serum, abscess, and tonsil. Further, the concentration in affected tonsil was significantly lower than that in unaffected tonsil. Moreover, the concentrations in serum were positively correlated with that in abscess, affected tonsil, and unaffected tonsil. The correlation coefficient was highest between serum and unaffected tonsil and lowest between serum and abscess. Those results indicate that surgical drainage such as puncture and incision of the abscess is essential for the treatment of peritonsillar abscess and that poor penetration of antibiotics into affected tonsil might be a reason why peritonsillar abscess occurs unilaterally. Further it is suggested that antibiotics having not only high sensitivity but also good tissue penetration should be used for peritonsillar abscess after surgical treatments and for acute tonsillitis to reduce the occurrence of peritonsillar abscess.