Patient 1 was an 82-year-old woman without symptoms who had been admitted to the hospital because of abnormal shadows on a chest X ray film. The chest X ray showed a round opacity in the right middle lung field and air-space consolidation in the right lower lung field. A computed tomographic (CT) scan showed a nodular opacity with satellite lesions in the right middle lobe and bronchiectasis in the right middle lobe and left lingular segment. Bronchial lavage yielded Mycobacterium intracellulare. Patient 2 was the 77-year-old sister of Patient 1. She had productive cough and fatigue, and chest X ray films and CT scans showed air-space consolidation with a huge cavity in the left upper lobe. M. intracellulare was isolated repeatedly from sputum samples. Pulsed-field gel electrophoresis disclosed similar large restriction fragment patterns of genomic DNA in 2 strains of the bacterium isolated from both sisters, indicating that the sisters had been infected by genetically close strains. Cross-infection or infection from the same environmental source were suspected.