A 66-year-old man was admitted to Nara Medical University Hospital because of sputum production and fevre. A chest X-ray film obtained on admission revealed many cysts and an infiltrative shadow in the right upper lung field. The patient was treated with antimycobacterial drugs (isoniazid 400 mg, streptomycin 0.75 g, and rifampicin 450 mg) because acid-fast bacilli were detected in his sputum. Although the symptoms and laboratory data improved, a new infiltrative shadow developed in the right lower lung field two months after the start of treatment. Transbronchial biopsy specimens showed intraluminal organizing exudate and alveolitis. The new lesion resolved when treated with the same antimycobacterial drugs. Mycobacterium xenopi was cultured from the sputum 80 days later. This is the third reported case of atypical mycobacteriosis (non-tuberculous mycobacteriosis) due to M. xenopi in Japan with the "initial aggravation" seen in some patients with typical pulmonary tuberculosis.