A comparative clinical study of lomefloxacin hydrochloride (LFLX) was performed to confirm the optimal dose of LFLX for treatment of respiratory tract infections. Daily dosages of LFLX, 400 mg (200 mg b.i.d.) and 600 mg (200 mg t.i.d.) were allocated by the randomization method. Treatment period was 7-14 days. Of a total of 136 cases, 14 cases were excluded from evaluation of clinical efficacy and 4 cases from evaluation of safety. There were no differences in the background factors of the patients between the 2 dosage groups. The clinical efficacy rates, judged by a committee, were 76.4% (42/55) for the 400 mg group and 74.6% (50/67) for the 600 mg group. No significant difference was observed between the 2 groups (P = 1.000). Incidences of adverse effects were 1.6% (1/64) for the 400 mg group and 2.9% (2/68) for the 600 mg group. Incidences of abnormal laboratory test values were 3.4% (2/58) for the 400 mg group and 1.6% (1/63) for the 600 mg group. No significant differences were found between the 2 groups. These adverse effects and abnormal laboratory test values improved with the cessation of the administration of the drug. From these results, a dose of 400 mg (200 mg b.i.d.) LFLX seems to be better than that of 600 mg (200 mg t.i.d.) LFLX in the treatment of respiratory tract infections.