The cochlea of the mammalian inner ear contains three rows of outer hair cells and a single row of inner hair cells. These hair cell receptors reside in the organ of Corti and function to transduce mechanical stimuli into electrical signals that mediate hearing. To date, the molecular mechanisms underlying the maintenance of these delicate sensory hair cells are unknown. We report that targeted disruption of Barhl1, a mouse homolog of the Drosophila BarH homeobox genes, results in severe to profound hearing loss, providing a unique model for the study of age-related human deafness disorders. Barhl1 is expressed in all sensory hair cells during inner ear development, 2 days after the onset of hair cell generation. Loss of Barhl1 function in mice results in age-related progressive degeneration of both outer and inner hair cells in the organ of Corti, following two reciprocal longitudinal gradients. Our data together indicate an essential role for Barhl1 in the long-term maintenance of cochlear hair cells, but not in the determination or differentiation of these cells.