Purpose: In this study, the authors assessed age- and sex-related patterns in the prevalence and 10-year incidence of 4-kHz air-bone gaps and associated factors. Method: Data were obtained as part of the longitudinal, population-based Epidemiology of Hearing Loss Study ( Cruickshanks et al., 1998). An air-bone gap at 4 kHz was defined as an air-conduction threshold 15 dB higher than the bone-conduction threshold in the right ear. Results: Among 3,553 participants ages 48-92 years at baseline (1993-1995), 3.4% had a 4-kHz air-bone gap in the right ear. The prevalence increased with age. Among the 120 participants with an air-bone gap, 60.0% did not have a flat tympanogram or an air-bone gap at 0.5 kHz. Ten years later, the authors assessed 2,093 participants who did not have a 4-kHz air-bone gap at baseline; 9.2% had developed a 4-kHz air-bone gap in the right ear. The incidence increased with age. Among the 192 participants who had developed an air-bone gap, 60.9% did not have a flat tympanogram or air-bone gaps at other frequencies. Conclusion: These results suggest that a finding of a 4-kHz air-bone gap may reflect a combination of aging and other factors and not necessarily exclusively abnormal middle-ear function.