AbstractBackgroundStudies have reported significant associations between asthma and attention‐deficit/hyperactivity disorder (ADHD), but whether the association is due to shared etiology such as shared genetic risk factors remains unclear. We aimed to investigate patterns of familial co‐aggregation of asthma and ADHD and also to quantify the relative contribution of genetic and environmental influences.MethodsThrough Swedish register linkages, we obtained a cohort of 927,956 individuals born 1992–2001 and identified monozygotic twins (MZ), dizygotic twins (DZ), full‐ and half‐siblings, and full‐ and half‐cousins. Clinical diagnosis of asthma and ADHD were identified from the Swedish national registers. We used logistic regressions to investigate the within‐individual association and familial co‐aggregation between asthma and ADHD. We then used bivariate twin modeling to quantify the genetic and environmental correlations and their contributions to the familial liability.ResultsIndividuals with asthma had significantly higher risk of ADHD (odds ratio [OR], 1.50; 95% confidence interval [CI], 1.47–1.54). Relatives of individuals with asthma had an increased risk of ADHD compared to relatives of individuals without asthma; in familial co‐aggregation analysis, the association was strongest in MZ twins (OR, 1.67; 95% CI, 0.99–2.84) and attenuated with degree of genetic relatedness. In the twin modeling, the phenotypic and genetic correlations between asthma and ADHD estimated from the ACE model were 0.09 (95% CI, 0.05–0.14) and 0.12 (95% CI, 0.02–0.21), respectively. The bivariate heritability was 0.88 (95% CI, 0.30–1.46). Estimates for contributions from shared and non‐shared environment factors were not statistically significant.ConclusionsAsthma and ADHD co‐aggregate in families primarily due to shared genetic risk factors. Within‐individual and family history of either disorder should prompt clinical assessment of the other condition. Future studies should further investigate genetic variants underlying the co‐occurrence of ADHD and asthma.