The effectiveness and safety of cefaclor advanced formula (AF) compared with cefaclor in the treatment of acute exacerbations of chronic bronchitis were evaluated in multicentre trials in the United States and Europe. In the US study, patients were randomized to receive either cefaclor AF 375 mg twice daily or 500 mg twice daily or 250 mg cefaclor three times daily. Three dose levels of cefaclor AF were used in the European study: 375 mg, 500 mg and 750 mg twice daily; the comparative agent was cefaclor 250 mg three times daily in the 375 mg and 500 mg strata, and cefaclor 500 mg three times daily in the 750 mg stratum. The total number of patients receiving cefaclor AF was 667 versus 357 patients receiving cefaclor. Results showed that cefaclor AF administered twice daily at 375, 500 or 750 mg was as effective as cefaclor 250 mg three times daily for the treatment of acute exacerbations of chronic bronchitis. Evaluable patients receiving cefaclor AF in the US study had a clinical response rate of over 90% and a bacteriological response rate of over 85%. In the European study, patients receiving cefaclor AF had clinical response rates over 85% and bacteriological response rates ranging from 50% to 85%. The safety profile for cefaclor AF was similar to that reported for other beta-lactam antibiotics.