Findings from this US cross-sectional study showed that familial dilated cardiomyopathy (DCM) had a substantial prevalence among patients, and their first-degree family members had increased lifetime risk of DCM.

• In this family-based, cross-sectional study of 1,220 patients with DCM and their 1,693 family members, the prevalence of familial disease among patients with idiopathic DCM and the lifetime risk of DCM for their first-degree family members by race and ethnicity was estimated.

• The estimated familial DCM prevalence was 29.7%.

• Black probands had a higher estimated prevalence of familial DCM compared with White probands (difference, 11.3%), but estimated prevalence did not differ significantly between Hispanic probands and non-Hispanic probands (difference, −1.4%).

• The DCM risk by age 80 years in family members was estimated to be 19%.

• A higher DCM hazard was observed in first-degree relatives of non-Hispanic Black probands than non-Hispanic White probands (hazard ratio, 1.89).