Donor T lymphocytes injected into the host systemic circulation were observed to infiltrate into the host endolymphatic sac in mice. These findings suggest that the endolymphatic sac, a major immune organ in the inner ear, is supplied with immunocompetent cells from the systemic circulation. This concept is consistent with clinical reports that inner ear disorders accompany certain systemic autoimmune diseases. Bone marrow transplantation to replace autoreactive immunocompetent cells with normal cells should be considered as a potential therapy for inner ear autoimmune diseases and an alternative to conventional treatments.