Two prospective randomized, double-blind, parallel studies were carried out in Europe to compare cefaclor advanced formulation (cefaclor AF) with cefaclor in the treatment of acute bronchitis caused by susceptible pathogens. A total of 1,321 patients suffering from acute bronchitis confirmed by clinical data and a negative chest X-ray were randomized for treatment in the two multicentre trials. Three doses of cefaclor AF were tested: 375 mg twice daily and 500 mg twice daily were compared with cefaclor 250 mg three times daily; and cefaclor AF 750 mg twice daily was compared with cefaclor 500 mg three times daily. Duration of therapy was seven days. Assessments (complete history, physical examination, sputum specimens for culture and Gram's stain, plus clinical and laboratory evaluations of safety) were carried out within 24 hours before the first dose, during therapy, within 72 hours after therapy completion and, in the 375 mg and 500 mg dose groups, 1-2 weeks after the end of therapy. There were no significant differences between the total evaluable cefaclor AF population and the total evaluable cefaclor population with regard to favourable post-therapy responses. Most favourable clinical and bacteriological response rates in the 375 and 500 mg doses were 80% or above. In the higher dose group, there was a favourable post-therapy symptomatic response in 100% of evaluable patients, with favourable bacteriological responses in 93.3% patients receiving cefaclor AF and 96.8% receiving cefaclor (no significant difference). Only one serious drug-related adverse event was reported (anaphylactic reaction). Adverse events related to the digestive system were reported by 4.7% of the cefaclor AF-treated patients and 4.5% of the cefaclor-treated patients during the entire study period. Cefaclor AF, at all three dose levels studies, was seen to be as safe as cefaclor in the treatment of acute bronchitis caused by Streptococcus pneumoniae, Haemophilus influenzae and Moraxella (Branhamella) catarrhalis.