In this randomised, double-blind study carried out in 28 centres, azithromycin (500 mg single dose on day 1, followed by 250 mg once-daily on days 2-5) was compared with cefaclor (500 mg t.i.d. for 10 days) in the treatment of acute bacterial pneumonia. A total of 119 patients entered the study, and of these 71 were evaluable and included in the efficacy analysis. The overall satisfactory clinical response was 97.3% for azithromycin patients and 100% for cefaclor patients. The clinical cure rates of azithromycin and cefaclor were 46.9% and 41.0%, respectively; improvement was seen in an additional 46.9% of azithromycin-treated patients and in 59.0% of the cefaclor group. The bacteriological eradication rates were 80.4% and 92.6%, respectively. These rates of clinical and bacteriological efficacy, were not statistically different. Both antibiotics were well tolerated during the study; only two patients (one on each study drug) discontinued medication due to adverse events. The overall incidence of side effects was 18.9% (10 of 53 patients) for azithromycin- and 12.1% (eight of 66 patients) for cefaclor-treated patients. Gastrointestinal disturbances were the most commonly reported side effects (nine of 10 azithromycin-treated patients and six of eight cefaclor-treated patients). In addition, two cefaclor patients reported headache. All azithromycin side effects were mild or moderate in severity, but there were two severe occurrences in the cefaclor group (1 nausea, 1 vomiting) the later leading to discontinuation.(ABSTRACT TRUNCATED AT 250 WORDS)