A 43 year-old Japanese male was admitted to our hospital because of productive cough and fever. He was diagnosed as acquired immunodeficiency syndrome (AIDS) in 1994. Laboratory findings were as follows: WBC was 3200/microliter, CD4+ T lymphocyte count was 22/microliter. His chest X-ray film taken on admission showed infiltration with small cavity lesion in middle left lung field. Tuberculin skin reaction was negative. He was treated with isoniazid 0.4 g, rifampicin 0.45 g, and ethambutol 0.75 g each daily. Sputum smear was positive for acid fast bacilli. The cultured isolates were identified as Mycobacterium kansasii (M. kansasii) and Mycobacterium avium complex (MAC). Urine smear was also positive for acid fast bacilli. The cultured isolates were identified as M. kansasii. He was diagnosed as disseminated M. kansasii infection and suspected MAC infection. About one hundred days later, his chest X-ray film showed reticular shadow. His clinical symptoms improved and the sputum smear and culture converted to negative for acid fast bacilli. Based on these findings, his MAC discharge was considered not as MAC infection, but MAC colonization. He returned to the former hospital for AIDS treatment, and he died in August 1996.