The results of a Danish nationwide cohort study involving almost two million children followed up from birth to age 16 years reveal excess morbidity in children whose parents have rheumatoid arthritis (RA). Detrimental effects occur in children exposed to paternal as well as maternal RA, suggesting a genetic (rather than epigenetic) aetiology. Most of the 11 disease categories assessed show similar risk increases, but the largest risk increases are observed for autoimmune diseases: a threefold increase in risk of juvenile idiopathic arthritis (HR 3.30, 95% CI 2.71–4.03 for maternal RA and HR 2.97, 95% CI 2.20–4.01 for paternal RA); a ≤40% increase in the risk of type 1 diabetes mellitus (HR 1.37, 95% CI 1.12–1.66 and HR 1.44, 95% CI 1.09–1.90) and a ≤30% increase in the risk of asthma (HR 1.28, 95% CI 1.20–1.36 and HR 1.15, 95% CI 1.04–1.26), respectively.