In a prospective, randomised, nonblinded, multi-investigator trial, cefprozil 500mg twice daily was compared with cefaclor 500mg 3 times daily, both administered orally for 10 days for episodes of bronchitis. A total of 247 patients aged 65 years or older with mild to moderate acute bronchitis or exacerbations of chronic bronchitis were treated in 30 in- and outpatient clinical sites in Europe and North America. Of these, 104 patients did not have a defined bacterial pathogen in pretreatment sputum cultures. Among the 143 other patients,Haemophilus influenzae, Streptococcus pneumoniaeandMoraxella (Branhamella) catarrhaliswere the most frequently isolated bacteria. There was a satisfactory clinical response to treatment in 119 of 156 cefprozil-treated patients (84%) and in 63 of 91 (82%) of those treated with cefaclor. Eradication of the presumed bacterial pathogen occurred in 74 of 89 (83%) cefprozil-treated patients and 35 of 42 (83%) cefaclor-treated patients. Both cefprozil and cefaclor were well tolerated, with possible drug-related adverse reactions in only 16 of 156 (10%) cefprozil-treated patients and 10 of 91 (11%) cefaclor-treated patients. Thus, in these clinical trials cefprozil was shown to be a safe, well-tolerated and effective treatment for episodes of bronchitis in elderly patients.