In a single-blind study, 134 patients with bronchopneumonia or lobar pneumonia were randomly assigned to receive 400 mg of loracarbef twice daily or 500/125 mg of amoxicillin/clavulanate three times daily for 10 to 14 days. Treatment efficacy was evaluated in 38 patients treated with loracarbef and in 39 treated with amoxicillin/clavulanate in whom pre-treatment positive cultures of pathogens susceptibile to the study drugs were isolated. Streptococcus pneumoniae and Haemophilus influenzae were cultured as single pathogens in 40.3% of the evaluable patients. Among the evaluable patients, 100% of the loracarbef group and 92.3% of the amoxicillin/clavulanate group had a favorable clinical response (cure or improvement). Bacteriologic response was favorable and the pathogen was eliminated or presumed eliminated in 97.4% of the loracarbef-treated patients and in 92.3% of the amoxicillin/clavulanate-treated patients. Treatment was discontinued in one loracarbef-treated patient because Ludwig's angina, unrelated to the study drug, was diagnosed and in five amoxicillin/clavulanate-treated patients because of diarrhea (two patients), rash (two patients), and nausea and vomiting (one patient). Diarrhea, the most frequently cited adverse event, was reported by 6.1% of the loracarbef-treated patients and 11.8% of the amoxicillin/clavulanate-treated patients. Asthenia was reported by 0% and 8.8% of the loracarbef and amoxicillin/clavulanate patients, respectively. It is concluded that both loracarbef and amoxicillin/clavulanate are safe and effective in the treatment of acute bacterial pneumonia.