In this study the efficacy of bacampicillin and amoxicillin in treatment of bacterial lower respiratory tract infection were compared. Thirty-eight patients were treated with bacampicillin (800 mg twice a day), and 39 were treated with amoxicillin (500 mg three times a day). Conditions treated included pneumonia, exacerbation of chronic bronchitis, and bronchiectasis. The two groups were roughly comparable in terms of demographic factors and clinical diagnosis. The most common pathogens were Streptococcus pneumoniae and Hemophilus influenzae; isolates of S. pneumoniae were somewhat more prevalent in the group treated with bacampicillin. All patients in both groups were either cured or improved clinically. The offending pathogen was eliminated except for two strains of H. influenzae in the group treated with amoxicillin. There were mild adverse effects, including two cases of diarrhea, in four patients treated with amoxicillin. Two patients treated with bacampicillin had mild adverse effects; no diarrhea was encountered. Minor abnormalities of laboratory test results that possibly were related to therapy were encountered in eight of the patients treated with bacampicillin and three of the patients treated with amoxicillin.