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Heritability of OSA in a Rural Population. BACKGROUND: OSA has a familial aggregation pattern indicating that it can be partially caused by a genetic component. However, the heritability of OSA has been estimated based on the study of families of obese probands of urban populations with established OSA diagnosis. The objective of this genetic-epidemiologic study is to study families ascertained from a general rural population to determine an unbiased estimate of OSA heritability. METHODS: We studied a sample of families living in Baependi, a small rural southeastern Brazilian city. Participants were assessed for anthropometric measurements, physical examination, Epworth Sleepiness Scale, blood samples for glucose and cholesterol determination, and overnight home portable monitoring. RESULTS: We studied 587 participants (399 women) from 91 families, with a median (interquartile range [IQR]) of 4 (2-8) participants per family. The median age of the population was 44 years (IQR, 29-55 years) and median BMI was 25.0 kg/m(2) (IQR, 22.1-28.6 kg/m(2)). OSA, defined by apnea-hypopnea index (AHI) > 5/h, was diagnosed in 18.6% of the sample. Two polygenic models, model I (no covariate effects) and model II (with covariate effects), were fitted to the data in all analyses. Heritability estimates for AHI were 0.23 and 0.25 for model I and II, respectively. Covariates (age, sex, and BMI) showed no significant effects on the heritability estimate for AHI. CONCLUSIONS: The heritability of AHI in a rural population with low levels of obesity is intermediate (25%).