Three cases of pulmonary atypical mycobacteriosis (AM) were reported. Two cases were associated with lung cancer in which the diagnosis of malignancy was difficult and delayed by the coexistence of AM. The third was a case of adult T-cell leukemia (ATL) which manifested during the course of AM. In case 1 (73 years, male) and case 2 (86 years, male), chest roentgenogram abnormalities as well as clinical symptoms were considered to be caused by mycobacteriosis because of positive smear of acid-fast bacilli in sputa on admission. Therefore it took four months and three months respectively for final diagnosis of lung cancer. The autopsy of case 1 revealed a poorly differentiated adenocarcinoma with coexisting foci of squamous cell carcinoma in right lower lung, and granulomatous inflammations with caseating necroses in right mid and lower lungs. M. avium complex was cultured from sputum on admission, and also a high titer of HTLV-I antibody was demonstrated. In case 2 malignant cells were detected in sputa (class V), however his general condition did not allow an aggressive anticancer chemotherapy and he died of malignancy with complication of thromboangiitis obliterans on right lower leg. Case 3 was a 76-year-old male who had been diagnosed as lung AM for more than two years. His chest radiography showed bilateral infiltrative shadows with frequent positive cultures of M. avium complex (more than 100 colonies) from sputum. A generalized lymphadenopathy including right hilar lymph node on chest X-ray film was followed by the presence of atypical lymphocytes in peripheral blood and the elevation of HTLV-I antibody in serum. Four months later he died with hypercalcemia and renal failure in spite of chemotherapy (CPM + VCR + ADR + PLS). The above cases suggest that AM as well as tuberculosis should be considered when pulmonary infiltrates were observed in malignant patients, especially in patients with retrovirus infections.