The efficacy and tolerance of clarithromycin and erythromycin stearate in the treatment of community-acquired pneumonia were compared in a multicentre, double-blind randomized trial. Two hundred and eight adult patients were randomized to receive either clarithromycin 250 mg 12-hourly (96 patients) or erythromycin stearate 500 mg 6-hourly (112 patients), each for 14 days. One hundred and eight patients were evaluable for efficacy, 64 receiving clarithromycin and 44 erythromycin stearate. There was no significant difference between the two groups in terms of clinical cure (52% for clarithromycin, 40% for erythromycin) or clinical success (clinical cure and improvement; 89% for clarithromycin, 98% for erythromycin stearate), or radiological response (90% for both groups). An intention-to-treat analysis, including all patients entering the study revealed significant differences in favour of clarithromycin. The clinical cure rate after two weeks of treatment was 45% in those who received clarithromycin compared with 25% in the erythromycin stearate group (P = 0.003), whilst improvement in cough was observed in 97% and 80% of patients receiving clarithromycin and erythromycin stearate, respectively (P = 0.07). Adverse effects, mainly gastrointestinal, caused discontinuation of treatment in 4% (4/96) patients in the clarithromycin group in comparison with 19% (21/112) treated with erythromycin stearate (P less than 0.01). These results demonstrate that clarithromycin twice daily is at least as effective as four times daily erythromycin stearate for the treatment of community-acquired pneumonia and is better tolerated.