OBJECTIVES: To estimate sibling risk of hospitalization for children with sleep disordered breathing (SDB), diagnosed with (1) obstructive sleep apnea syndrome (OSAS), or (2) adenotonsillar hypertrophy in the total Swedish population. DESIGN, SETTING, AND PARTICIPANTS: Using the MigMed database at the Karolinska Institute, we divided the population of Sweden aged 0-18 years into sibling groups based on a shared mother and father and presence of a primary hospital diagnosis of OSAS or adenotonsillar hypertrophy for each individual born between 1978 and 1986, during the follow-up period 1997-2004. Individuals with at least one affected sibling were identified and the incidence rates were computed, using standardized incidence ratios (SIRs) with 95% confidence intervals (CIs). Reference groups were boys and girls with unaffected siblings of 2 or more. RESULTS: After accounting for socioeconomic status, age, and geographic region, boys with at least one sibling with OSAS had an increased risk of having OSAS (SIR, 33.2; 95% CI, 16.5-64.8), and in girls the SIR was 40.5 (19.4-81.4). For hypertrophy of the tonsils or hypertrophy of the adenoids and tonsils the corresponding SIRs were 4.53 (3.0-6.8) for boys and 4.94 (3.3-7.4) for girls. CONCLUSIONS: The study indicate an increased sibling risk of sleep disordered breathing in children, which may be due to heritable genes and/or shared environment such as increased awareness among family members or referring doctors. Caregivers should ask parents if siblings have similar symptoms, and thus offer them early treatment.