Objectives To examine the familial risk of systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), including juvenile rheumatoid/idiopathic arthritis (JRA), in a population-based setting; and to determine whether patterns of transmission differ according to the sex of the parent or offspring, in order to provide insight into the potential impact of X-chromosomal factors on sex disparities in these autoimmune diseases.

Methods A population-based cohort of parent–offspring triads from Denmark (1977–2010) was established. SLE and RA incidence rates among offspring were calculated, and Cox regression was performed to assess the sex-specific risk of disease in offspring according to maternal or paternal disease history.

Results Among 3 513 817 parent–offspring triads, there were 1258 SLE cases among offspring (1095 female, 163 male) and 9118 cases of RA/JRA (6086 female, 3032 male). Among female offspring, SLE risk was nearly the same according to maternal (HR 14.1) or paternal (HR 14.5) history (p=NS); likewise among male offspring, risk according to maternal (HR 5.5) and paternal (no cases) history were similar (p=NS). For RA, all risk estimates were similar, regardless of the sex of the offspring or parent (HR 2.6–2.9; p=NS).

Conclusions The authors quantified the familial risk of SLE and RA in a nationwide cohort study. For both diseases, transmission was comparable among both female and male offspring of maternal and paternal cases. These data provide evidence at the population level that X-chromosomal factors do not play a major role in sex disparities associated with the risk of SLE and RA.