Objective: To estimate the influence of parental rheumatoid arthritis (RA) on child morbidity.

Design: Nationwide cohort study.

Setting: Individual linkage to nationwide Danish registries.

Participants: All singletons born in Denmark during 1977-2008 (n=1 917 723) were followed for an average of 16 years.

Main outcome measures: Adjusted HRs for child morbidity; that is, 11 main diagnostic groups and specific autoimmune diseases within the International Classification of Diseases 8th and 10th versions.

Results: Compared with unexposed children, children exposed to maternal RA ('clinical' and 'preclinical') (n=13 566) had up to 26% higher morbidity in 8 of 11 main diagnostic groups. Similar tendencies were found in children exposed to paternal RA ('clinical' and 'preclinical') (n=6330), with statistically significantly higher morbidity in 6 of 11 diagnostic groups. HRs were highest for autoimmune diseases with up to three times increased risk of juvenile idiopathic arthritis (HR, 95% CI 3.30, 2.71 to 4.03 and 2.97, 2.20 to 4.01) and increased risk of up to 40% of diabetes mellitus type 1 (HR, 95% CI 1.37, 1.12 to 1.66 and 1.44, 1.09 to 1.90) and up to 30% increased HR of asthma (HR, 95% CI 1.28, 1.20 to 1.36 and 1.15, 1.04 to 1.26). Conclusions were roughly similar for children exposed to maternal clinical RA and for children only followed up to 16 years of age.

Conclusion: Children of parents with RA had consistent excess morbidity. If the associations reflect biological mechanisms, genetic factors seem to play an important role. These findings call for attention given to children of parents with RA.