Sixty seven cases with pulmonary atypical mycobacteriosis (2 by M. kansasii, 62 by M. intracellulare, 2 by M. fortuitum and 1 by a possible new pathogen of group III mycobacteria) died in 9 Japanese national sanatoria up to October 1976. Of these 67 cases, 40 died of atypical mycobacterial disease, and the remaining 27 cases died of other diseases. Roentgenological aggravation was found in 44 cases (1 by M. kansasii, 1 by M. fortuitum and 42 by M. intracellulare). There were various types of roentgenological aggravation. Spread of non-cavitary foci, infiltrate and pneumonia were found most frequently (40/44, 90.9%). Enlargement of cavity was found in 12 cases (27.2%), appearance of pleural effusion in 5 cases (11.4%), and spontaneous pneumothorax in 3 cases. Infection of bulla was found in 11 out of 22 cases with bullae as the underlying disease. The first roentgenological aggravation was found in 20 (45.5%) out of 44 cases within 12 months; 13 cases (29.5%) between 13 to 24 months; and 11 cases (25%) over 2 years after the discovery of the disease. From the results mentioned above, in the fatal cases, progression of the disease was predicted by the appearance of the roentgenological aggravation within 2 years after the discovery of the disease. There were various courses of the progression of the lesions as shown in Figures 2a and 2b. One of the typical course of the progression was devided into the following 5 stages: The first stage: localized cavitary lesion. The second stage: spread of foci around cavity. The third stage: spread of foci in contralateral lung. The fourth stage: enlargement of cavity (appearance of giant cavity). The fifth stage: extensive pneumonia in the lower lung field. Another typical course of the progression was the repeated infections of bullae. Roentgenological aggravation found in patients with atypical mycobacterial disease was not rarely due to the mixed infection with various organisms (gram-negative bacilli, fungi and also human type tubercle bacilli). The majority of the patients with underlying pulmonary disease (extensive emphysema, chronic bronchitis and bronchiectasis) died of pulmonary insufficiency in the relatively early stage of atypical mycobacteriosis. The patients with the mixed infection have died, in spite of the negative conversion or the dicrease of the excretion of atypical mycobacteria. There were two cases (M. intracellulare infection) complicated with pulmonary tuberculosis.