Responses to the eight-item Epworth Sleepiness Scale (ESS) obtained from 1560 World War II male veteran twin pairs [818 monozygotic (MZ), 742 dizygotic (DZ)] were analysed to determine the extent to which genetic influences are involved in self-reported daytime sleepiness in the elderly. Average ESS score (+/- SD) in this sample was 7.1 +/- 3.9, range 0--24. More than half of the twins (65%--67%) reported a moderate to high chance of falling asleep while lying down to rest; fewer than 3% admitted that this would occur while sitting and talking to someone or while stopped in traffic. Daytime sleepiness was not associated with age but was significantly and positively associated with obesity. The intraclass twin correlation on ESS scores was 0.39 in MZ pairs and 0.21 in DZ pairs (both P < 0.001). Structural equation modeling of the observed variance-covariance matrices for MZ and DZ twins estimated the heritability of ESS to be 38% (95% confidence interval 33%--44%). Environmental influences not shared by twin brothers accounted for the remaining variance in daytime sleepiness. A reasonable interpretation of the heritability of ESS in this healthy cohort of elderly male twins is a genetic susceptibility for disordered breathing during sleep.