Long-term erythromycin therapy has been reported as effective in the treatment of various chronic respiratory diseases, including chronic sinusitis. We investigated the clinical efficacy of long-term administration of the erythromycin derivative clarithromycin (CAM) in intractable cases of chronic sinusitis. Forty-five adult patients were treated with dosages of 400 mg/day for 8 to 12 weeks. The results were as follows. i) Improvement of symptoms and rhinoscopic findings was noted in 71.1% of the patients. ii) For periods of up to 12 weeks, clinical efficacy depended upon the duration of treatment. Administration for more than 12 weeks might further improve clinical results. iii) No significant side effects were noted during the course of CAM treatment. On the whole, CAM was at least as effective as erythromycin in the treatment of chronic sinusitis.