The clinical effectiveness and side effects of DL-8280 in the treatment of lower respiratory tract infections including pneumonias were compared with those of amoxicillin (AMPC) by a double-dummy fashion. The following diseases were included in this study; infectious exacerbation of chronic bronchitis and diffuse panbronchiolitis, chronic respiratory tract diseases with infections (bronchiectasis, bronchial asthma, pulmonary emphysema, lung fibrosis, inactive pulmonary tuberculosis) and various kinds of bacterial pneumonias. Two hundred mg dose of DL-8280 was orally administered 3 times per day and 250 mg dose of AMPC was orally administered 4 times per day. Out of all 279 patients, DL-8280 was used in 139patients and AMPC was in 140 patients. Clinical effectiveness and side effects were observed and the obtained results were as follows. 1) One hundred and twenty-two patients of DL-8280 group and 115 patients of AMPC group were evaluated on clinical effectiveness. In DL-8280 group, 6 patients were judgedas “Excellent”, 89 as “Good”, 16 as “Fair”, 10 as “Poor” and its effectiveness rate was 78.5%. On the other hand, in AMPC group, 5 patients were judged as “Excellent”, 73 as “Good”, 14 as “Fair”, 22 as “Poor” and the effectiveness rate was 68.4%. However, one unevaluable patient was seen in each group. The clinical effectiveness of DL-8280 was superior to that of AMPC, especially, the effectiveness rate of DL-8280 group was significantly higher in chronic bronchitis. 2) Bacteriologically, the eradication rate of 88.6% was observed out of 79 clinical isolates in DL-8280 group and it was 61.1% in 72 clinical isolates in AMPC group. These results revealed the significant superiority of DL-8280 to AMPC. Particularly, the bacteriological response of DL-8280 against H. influenzae was significantly superior to that of AMPC. 3) Side effects were noted in 6.1% of 132 patients with DL-8280 and 7.7% of 130 patients with AMPC, and abnormal changes in laboratory findings were noted in 11.1% in DL-8280 group and 14.2% in AMPC group after administration. There was no significant difference between two groups. Neither progressive side effect nor remarkable change in laboratory findings was noted. 4) From these results, it was concluded that DL-8280 was more useful agent than AMPC in the treatment for lower respiratory tract infections, particularly chronic complicated cases.