A nationwide study was conducted to investigate the efficacy of Clarithromycin (CAM) on pulmonary atypical mycobacteriosis caused by the Mycobacterium avium-Mycobacterium intracellulare complex, including intractable cases. Out of total 97 patients examined, the analysis for bacteriological efficacy was possible in 69 cases. The negative conversion of bacilli was observed in 18 cases (26.1%), and 5 out of 12 cases in which the follow-up was conducted turned out continued negative status. The efficacy of CAM was relatively high in the following cases: the duration of the disease was less than 6 months the extent of pulmonary lesions on chest roentogenograms was limited or moderate; and antituberculous agents which were previously unused were applied in combination with CAM. Also, the efficacy was high in cases where the dose of CAM was 600 mg/day or higher. Major side effect was mill to moderate digestive symptoms. In conclusion, at daily dose of 600 mg or higher, CAM was effective in the elimination or reduction of M avium M. intracellulare complex that caused atypical mycobacteriosis, without developing serious side effect. Treatment with this drug should be attempted in intractable cases.