Medical treatment of pulmonary Mycobacterium avium complex (MAC) disease does not always provide curative effects and is frequently hampered by recurrence. This suggests the presence of a reservoir for MAC in the environment surrounding patients. We previously reported the recovery of MAC isolates from the residential bathrooms of outpatients. In the present study, to ascertain the colonizing sites and the possibility of an MAC reservoir in the bathrooms of patients, we tested the recovery and the genetic diversity of MAC isolates from 6 sites of specimens, including 2 additional sampling sites, inside the showerhead and the bathtub inlet, in the residential bathrooms of patients with pulmonary MAC disease. MAC isolates were recovered from 15 out of the 29 bathrooms (52%), including specimens from 14 bathtub inlets and 3 showerheads. Nearly half of these bathrooms (7/15) contained MAC strains that were identical or similar to their respective clinical isolates. Additionally, in 5 out of 15 bathrooms, polyclonal colonization was revealed by pulsed-field gel electrophoresis. The results imply that colonization of MAC organisms in the bathrooms of MAC patients occurs predominantly in the bathtub inlets, and there is thus a risk of infection and/or reinfection for patients via use of the bathtub and other sites in the bathroom.