ObjectivesTo investigate a potential shared susceptibility between rheumatoid arthritis (RA) and acute coronary syndrome (ACS) by estimation of the risk of ACS among full siblings of patients with RA.MethodsBy linking nation-wide Swedish registers, we identified a cohort of patients with new-onset RA 1996–2016, age- and sex-matched (5:1) general population comparator subjects, full siblings of RA and comparator subjects, and incident ACS events through 31 December 2016. We used Cox regression to estimate the HR of ACS among patients with RA and the siblings of patients with RA versus the general population, overall and stratified by RA serostatus. We explored the impact of traditional cardiovascular (CV) risk factors on the observed associations.ResultsWe identified 8109 patients with incident RA, and 11 562 full siblings of these. Compared with the general population, the HR of ACS in RA was 1.46 (95% CI 1.28 to 1.67) and 1.22 (95% CI 1.09 to 1.38) among their siblings. The increased risks seemed confined to seropositive RA (patients: 1.52 [1.30 to 1.79], their siblings: 1.27 [1.10 to 1.46]); no significant risk increase was observed among siblings of patients with seronegative RA (HR 1.13 [95% CI 0.92 to 1.39]). Adjustment for 19 traditional CV risk factors did not appreciably alter these associations.ConclusionSiblings of patients with RA are at increased risk of ACS, suggesting shared susceptibility between RA and ACS, indicating the need and potential for additional cardio-preventive measures in RA (and their siblings).