Acoustic injury results in destruction of hair cells and numerous nonsensory cells of the cochlea. How these injured structures undergo repair is not well understood. This study was designed to examine the cochlea for the presence of mononuclear phagocytes after tissue injury caused by noise damage. We used octave band noise (8--16 kHz) at three levels (106, 112, and 120 dB) for 2 hours and studied the mice at 1, 3, 7, and 14 days after noise exposure to determine how noise affected hearing thresholds, hair cell number, and tissue injury in the cochlea. Furthermore, we assessed the cochlea for presence of inflammation by performing immunohistochemistry for CD45, common leukocyte antigen. We counted the number of CD45(+) cells that were present in the cochlea at the above-mentioned time points after noise. CD45 is present on all bone marrow-derived white blood cells and is not otherwise expressed in the inner ear. We found that, after noise exposure, there is a large increase in CD45(+) cells. These marrow-derived cells are concentrated in the spiral ligament and spiral limbus, areas that are known to be susceptible to acoustic injury. It is possible that this inflammatory response plays a role in propagating cellular damage in these areas. Immunohistochemistry demonstrates that these cochlear cells are derived from the monocyte/macrophage lineage and serve a phagocytic function in the inner ear.