During the period from 1960 through Octover 1976, Mycobacterium kansasii was isolated from 72 patients in Japan.<BR>1) Sixty-seven patients were found to have parenchymal pulmonary disease due to this organism. Besides these cases, there were one patient with pleurisy and another with lupus dis seminatus miliaris faciae.<BR>2) In only 3 cases, the organism was isolated as saprophyte and showed no relation to active pulmonary disease (colonization). The patients with colonization were very rare in Japan up to the present.<BR>3) Fifty-eight patients (80.6%) were living in Tokyo and its vicinity, and only a few patients were found in other districts. Therefore, it is suggested that the incidence of the infec tion with M. kansasii is different geographycally in Japan.<BR>4) The majority (94.1%) of the disease occurred in men. The majority of the patients infected with M. intracellulare were in middle age and over and had some underlying lung diseases, while many patients infected with M. kansasii were found among younger personswithout any previous lung disease.<BR>5) On the chest radiogram of 64 pulmonary cases, cavitation was found in 55 cases (85.9%). The cavity was thin-walled and not rarely multiple, and was found more frequently in the mid lung than in the subpleural region.<BR>The lesions were found more frequently (3.4 times) in the right lung than in the left lung<BR>6) The sensitivity to rifampicin (RFP) (91.7% of the tested strain), ethionamide (TH) (90.2%), and cycloserine (CS) (94%) was a characteristic feature of the drug susceptibility of M. kansasii. Many strains were susceptible to viomycin (VM) (70.2%), capreomycin (CPM) (69.0%) and ethambutol (EB) (57.1%) and showed low grade resistance to isoniazid (INH) and EB.<BR>7) SM, PAS and INH was used most frequently. RFP, TH and CS was used in 48.4%, 35.9% and 32.8% of the patients respectively. Bacteriological conversion occurred in 54 patients (84.4%) by chemotherapy alone, and comparing by the regimen, the negative conversion occurred in 5 patients (55.6%) by the combination of SM, INH and PAS, in 11 patients (78.6%) by the chemotherapy including secondery drugs (EB, CS, VM, etc.) except RFP and TH, in 11 patients (91.7%) treated with the regimen containig TH, in 18 patients (94.7%) treated with the regimen containing RFP, and in 9 patients (100%) treated with the regimen containing both TH and RFP. By the chemotherapy excluding RFP and TH, less satisfactory results were obtained especially in the far advanced cases.<BR>8) Four patients received combined medical-surgical treatment (1 segmentectomy, 3 lobec tomy), and no complication was found.<BR>9) During the study period, 4 of 68 patients died, with 2 deaths attributed to M. kansasii infection, and the remaining 2 not attributed to M. kansasii infection.