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Immune checkpoint inhibitors (ICIs) are becoming widely used for the treatment of various types of cancer. However, characteristic side effects, which are referred to as immune-related adverse events, may appear, and they have important clinical implications for the management of patients treated with ICIs. The development of mycobacterial infections has also been reported, but they have mostly been seen in cases with tuberculosis, and only a few cases involved non-tuberculous mycobacteriosis. We herein present the case of an 82-year-old man who was treated with nivolumab for gastric cancer. After the 22nd course of the treatment, the patient experienced loss of appetite for 1 week, and infiltration shadows were observed in the lower lobe of the left lung. Treatment for bacterial pneumonia was ineffective, and the lung field shadow gradually worsened. Mycobacterium intracellulare was detected in two consecutive sputum cultures. Thus, the patient was diagnosed with Mycobacterium avium complex (MAC) lung disease, and treatment for MAC infection was thus initiated, with subsequent improvement of the patient's condition and infiltration shadows. At 7 months after the start of treatment, the sputum cultures became negative for acid-fast bacilli. Since MAC lung disease may develop acutely during immunotherapy with ICIs, clinicians should include it in the differential diagnoses for pneumonia during immunotherapy with ICIs. [ABSTRACT FROM AUTHOR]