This cross-sectional study recruited a population-based sample of young children from among the more than 8000 children in Massachusetts enrolled in the on-going Infant Care Practices Study. Stratified sampling was used to assemble a cohort of 250 children with habitual snoring (> 3 nights/week), 100 children with occasional snoring (<3 nights/week), and 100 non-snoring children. These children were studied within 3 months of their fourth birthday using overnight laboratory polysomnography and a detailed neurobehavioral evaluation, in order to test the hypothesis that, after adjusting for potential confounding variables, both parent-reported snoring and polysomnographic measures were associated with standardized measures of the following neurobehavioral domains: immediate attention, sustained attention, working memory, and symptoms of hyperactivity. These data also provided an opportunity to determine normal values for polysomnographic measures in 4-year-old children and to determine the prevalence of snoring and polysomnographic abnormalities among 4-year-old children.