We report a very rare case in which pulmonary non-tuberculous mycobacteriosis with Mycobacterium intracellulare deteriorated acutely, presumably because of pregnancy. A 36-year-old immunocompetent woman was referred to our hospital with a cough and a slight fever of 3 weeks' duration when she was 5 months pregnant. Chest radiography at the first visit showed multiple large thin-walled cavities in the left middle and lower lung fields. After antibiotic, cough-depressant and theophyllin treatment, her symptoms almost disappeared for a while although Mycobacterium intracellulare was detected in her sputum. One month later, however, her symptoms were aggravated and infiltrative shadows were observed around the cavities on chest radiographs. She was treated with clarithromycin as well as the antituberculous drugs rifampicin and ethambutol until delivery, and then streptomycin was added. Owing to these medications, her symptoms and the presence of Mycobacterium intracellulare in the sputum were being gradually eliminated. Radiographic improvement of the infiltrative shadows and slight reduction of the size of the cavities were also observed. However, no further improvement of the radiographic findings was observed, despite continued treatment. Finally, it was concluded that this disease may not be curable by such medication. The patient eventually underwent a left pneumonectomy about one year after the first visit.