A 38-year-old man with pancytopenia due to myelodysplastic syndrome exhibited persistent pyrexia and mediastinal lymphadenopathy. A biopsy specimen of the lymph nodes showed diffuse infiltration of epitheloid cells, including acid-fast bacilli identified as Mycobacterium kansasii. Cultures from gastric lavage and stool yielded M. kansasii, and granulomas were found in the bone marrow. A diagnosis of disseminated nontuberculous mycobacteriosis (DNTM) was established. The patient's condition improved after treatment with isoniazid, rifampicin, ethambutol and granulocyte colony-stimulating factor. Although DNTM due to M. kansasii is rare in Japan, it should be recognized as a possible cause of opportunistic infections.