In the present study, we used the envelope method to divide patients with respiratory infections into two groups: a monotherapy group given imipenem/cilastatin sodium (IPM/CS) and a combination therapy group given imipenem/cilastatin sodium plus amikacin sulfate (AMK). We then compared the clinical efficacy and safety between groups. 1. Safety was evaluated in 83 patients in the IPM/CS group and 88 in the IPM/CS + AMK group while clinical efficacy was evaluated in 77 and 80 patients in the respective groups. 2. The overall efficacy rate was 84.4% in the IPM/CS group. Among the main infections, the efficacy rates were 82.7% in 52 cases of pneumonia (including lung abscess), 100% in cases of infected bronchiectasis, 66.7% in six cases of secondary infection of chronic respiratory disease, and 100% in four cases of chronic bronchitis. The overall efficacy rate was 83.8% in the IPM/CS + AMK group. Among the main infections, the efficacy rates were 88.1% in 59 cases of pneumonia (including lung abscess), 83.3% in 12 cases of infected bronchiectasis, and 60.0% in five cases of secondary infection of chronic respiratory disease. No significant differences in efficacies were seen between groups. 3. In the IPM/CS group, the efficacy rates were 92.3% for patients without prior antibiotic therapy in the IPM/CS group and 68.0% for those with prior therapy; in the IPM/CS + AMK group, the respective rates were 83.7% and 83.9%. In the IPM/CS group, there was a significant difference in the responses of patients with and without prior antibiotic therapy (P < 0.05). 4. Side effects were observed in six patients in the IPM/CS group (7.2%) and two patients in the IPM/CS + AMK group (2.3%). Abnormal laboratory test results were noted in 5 patients in the IPM/CS group (6.0%) and in 10 in the IPM/CS + f1p4group (11.4%). There was no significant difference in the incidence of side effects between groups and no severe adverse reactions in either group. These results indicate that IPM/CS alone produces of good response in moderate to severe respiratory infections while IPM/CS combined with AMK is useful in intractable respiratory infections.