The efficacy and safety of azithromycin and clarithromycin in lower respiratory tract infection (LRTI) were compared in an open, multicentre study. Five hundred and ten adult patients with a diagnosis of LRTI, including acute bronchitis, acute infective exacerbations of chronic bronchitis (AIECB) or pneumonia were enrolled. The patients were randomly assigned to receive either azithromycin (n = 252) as a single daily dose of 500 mg for three days, or clarithromycin (n = 258) 250 mg twice daily for ten days. In AIECB patients, baseline comparisons of the two treatment groups showed that there were no differences in the number of previous episodes of infection or in the incidence of current or past smokers. The overall clinical efficacy was found to be similar in each treatment group on day 10 to 14, with a satisfactory response (cured or improved) in 94% of azithromycin- and 97% of clarithromycin-treated patients. At follow-up evaluation (day 18 to 22), 97% of azithromycin- and 100% of clarithromycin-treated patients who had improved at day 10 to 14, showed satisfactory outcomes. Bacteriological efficacy was similar in both treatment groups, with eradication of 100% vs 95% of isolates in the azithromycin and clarithromycin groups, respectively. In AIECB, 100% of pathogens were eradicated by azithromycin, although one patient was clinically assessed as failed. Clarithromycin eradicated 93% of pathogens in this group; all patients being assessed as cured or improved. Both drugs were well tolerated, with 9% and 6% of patients reporting adverse events with azithromycin and clarithromycin, respectively. These adverse events were largely gastrointestinal in origin. It was concluded that a three-day course of azithromycin is as effective and well tolerated as a ten-day course of clarithromycin in adults with acute LRTIs.