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Copper/zinc superoxide dismutase (Cu/Zn SOD) is a first‐line defense against free radical damage in the cochlea and other tissues. To determine whether deficiencies in Cu/Zn SOD increase age‐related hearing loss and cochlear pathology, we collected auditory brainstem responses (ABRs) and determined cochlear hair cell loss in 13‐month‐old 129/CD‐1 mice with (a) no measurable Cu/Zn SOD activity (homozygous knockout mice), (b) 50% reduction of Cu/Zn SOD (heterozygous knockout mice), and (c) normal levels of Cu/Zn SOD (wild‐type mice). ABRs were obtained by using 4‐, 8‐, 16‐, and 32‐kHz tone bursts. Cochleas were harvested immediately after testing, and separate counts were made of inner and outer hair cells. Compared with wild‐type mice, homozygous and heterozygous knockout mice exhibited significant threshold elevations and greater hair cell loss. Phenotypic variability was higher …