Background Normally, few immunocompetent cell are present in the endolymphatic sac (ES). During an active immune response in the inner ear, large amount of inflammatory cells, including immunocompetent cells, are seen in the ES. The current study aimed at assessing cellular proliferation within the ES during induced immune response in the inner ear. Methods Fifteen healthy, female SD rats were sensitized systemically with keyhole limpet hemocyanin (KLH), followed by local inoculation in the cochlea through basal turn fenestration with the same antigen. On Days 3, 7 and 14 following inoculation, the animal was sacrificed after intraperitoneal administration of 5-bromo-2 -deoxyuridine (BrdUrd), and the temporal bone harvested. Following decalcification, infiltration by BrdUrd- and IgG-positive cells in the ES was studied on frozen sections with H & E and immunohistochemical staining. Results During the secondary immune response in the inner ear against T-dependent antigens, there is increased cellular proliferation in the ES. The proliferated cells may differentiate into immunocompetent cells at the same location. Conclusions These findings indicate that the ES plays an important role in immune response of inner ear.