The clinical efficacy and tolerance of a three-day course of azithromycin was evaluated in patients with community-acquired pneumonia in an open, non-comparative pilot study. Sixty-six patients with clinical and radiological findings of pneumonia were treated with a total dose of 1.5 g azithromycin (500 mg once-daily for three days). Of these 66 patients, 40 were evaluable clinically and microbiologically. Of the remaining 26 patients, 22 had no organisms isolated at baseline, and could only be evaluated clinically; two patients were lost to follow-up; and two patients were protocol violators. Of the 40 patients in whom aetiological agents were identified, 39 (98%) had a satisfactory clinical response (34 cured, five improved), including all eight patients diagnosed as having Legionella pneumophila pneumonia on the basis of serology. One patient with recurrent Haemophilus influenzae infection on day 8 was considered a failure. Total severity score for overall signs and symptoms was reduced significantly from a baseline level by day 2 (P = 0.0001). Bacteriological eradication was achieved in 32 of 33 (97%) isolates including 27 of 27 Streptococcus pneumoniae, the most frequently isolated pathogen. All six patients with pneumococcal bacteraemia were cured clinically by day 14, and blood cultures were negative for four of these patients within 48 h. Treatment-related side effects were reported in 4 of 66 (6%) patients, but were all mild. Laboratory abnormalities were observed in 5 of 62 (8%) patients: elevated liver enzymes (2), elevated creatine levels (1), haematological test abnormalities (2), including one patient with severe eosinophilia.(ABSTRACT TRUNCATED AT 250 WORDS)