||BACKGROUND: Hearing impairment is one of the most common permanent disabilities in the western world. Although hearing ability normally declines with age, there is great individual variation in age of onset, progression, and severity, indicating that individual susceptibility plays a role. The aim of the present study was to explore the relative importance of genetic and environmental effects in the etiology of impaired hearing. METHODS: From August 1995 to June 1997, the total adult population of Nord-Trondelag County, Norway, was invited to take part in the Nord-Trondelag Health Study. The survey included as an integrated project the Nord-Trondelag Hearing Loss Study with pure-tone audiometry assessment of the standard frequencies 0.25, 0.5, 1, 2, 3, 4, 6, and 8 kHz on 51,574 participants aged 20 to 101 years. We obtained information from Statistics Norway identifying 11,263 sibling pairs. After age stratification, we assessed similarity in hearing thresholds between siblings using polychoric correlations. The contribution of genetic effects in hearing ability was calculated. RESULTS: The upper limit of the heritability of hearing loss was 0.36. We found little evidence for sex differences in the relative importance of genetic effects. CONCLUSIONS: There is a substantial genetic contribution to individual variation in hearing thresholds.