BACKGROUND: It is poorly understood why some patients with atrial fibrillation develop heart failure (HF) and others do not. We examined the rate of developing HF in patients with atrial fibrillation with and without first-degree family members with HF or dilated cardiomyopathy (DCM). METHODS AND RESULTS: Using Danish nationwide registries, patients born after 1942 diagnosed with atrial fibrillation in the period 2005 to 2015 were identified and followed for up to 5 years. Patients with pre-existing HF, DCM, and/or ischemic heart disease diagnoses were excluded. Exposure was defined as a first-degree relative with HF or DCM. The rate of developing the composite end point of HF or death, and the components, was estimated with multivariable Cox proportional hazard regression models. We included 10 605 patients. A total of 17% had a family member with DCM/HF. Having a family member with HF/DCM was associated with an increased 5-year risk of the composite of HF/death (cumulative incidence, 9.2% [95% CI, 7.8–10.7] versus 5.6% [95% CI, 5.0– 6.1]; adjusted hazard ratio [HR] 1.36 [95% CI, 1.13–1.64]). (HF 8.4% [95% CI, 7.0– 9.8] versus 4.5% [95% CI, 4.1– 5.0]); (adjusted HR, 1.49 [95% CI, 1.22–1.82]). However, familial HF/DCM was not significantly associated with an increased 5-year risk and rate of death (0.8% [95% CI, 0.4–1.2] versus 1.1% [95% CI, 0.8–1.3]); (adjusted HR, 0.80 [95% CI, 0.46–1.39]). CONCLUSIONS: In patients with incident atrial fibrillation without prior ischemic heart disease or HF diagnoses, 1 of 6 had a first-degree relative with HF, and having such a family history of HF/DCM was associated with an 87% increase in 5-year incidence of HF compared with those without.