In this single-blind study, 488 patients with acute bronchitis were randomly assigned to receive 400 mg of loracarbef twice daily or 500/125 mg of amoxicillin/clavulanate three times daily for seven days. Treatment efficacy was evaluated in 98 patients treated with loracarbef and in 99 treated with amoxicillin-clavulanate in whom pretreatment positive cultures of pathogens susceptible to both study drugs were found. Streptococcus pneumoniae, Haemophilus influenzae, Moraxella (Branhamella) catarrhalis, and Klebsiella pneumoniae were isolated in pure or mixed cultures in 64% of the evaluable patients; S pneumoniae was found in 26%. Among the evaluable patients, the rate of favorable clinical responses (cure and improvement) in the loracarbef group (96 of 98 patients; 98.0%) was similar to that in the amoxicillin/clavulanate group (96 of 99 patients; 97.0%); the favorable bacteriologic response rates were also similar (93.7% vs 92.9%, respectively). Eight patients in the loracarbef group and nine in the amoxicillin/clavulanate group discontinued treatment because of adverse events. The events were presumed to be drug related in five of the loracarbef group and in seven of the amoxicillin/clavulanate group. During therapy, diarrhea was the most frequently reported event in both groups. However, it occurred in only 8.2% of the loracarbef-treated patients compared with 22.5% of the amoxicillin/clavulanate patients (P less than 0.001). It is concluded that both loracarbef and amoxicillin/clavulanate are safe and effective in the treatment of acute purulent bacterial bronchitis.