We conducted a case-control study in 100 cases who had Alzheimer's-type dementia and 100 age-, sex-, and education-matched, nondemented controls to evaluate the hypothesis that hearing impairment contributes to cognitive dysfunction in older adults. The prevalence of a hearing loss of 30 dB or greater was significantly higher in cases than in controls (odds ratio, 2.0; 95% confidence interval, 1.2 to 3.4), even when adjusted for potentially confounding variables. In addition, we observed a dose-response relationship in which greater hearing loss was associated with a higher adjusted relative odds of having dementia. Hearing loss was also significantly and independently correlated with the severity of cognitive dysfunction, as measured by the Mini-Mental State Examination, in nondemented as well as demented patients. These results demonstrate an association between hearing impairment and dementia and lend support to the hypothesis that hearing impairment contributes to cognitive dysfunction in older adults.