Introduction: Truncating TTN variants (TTNtv) play an important role in the development of dilated cardiomyopathy (DCM). We collected evidence for the pathogenicity of TTN c.59926+1G>A, studied its founder effect and describe the associated phenotype including the presence of (paroxysmal) atrial fibrillation ((p)AF) and also evaluated this in carriers of other TTNtv.Methods: Clinical and genetic data from 11 probands and 19 family members with the TTN c.59926+1G>A variant were collected. We studied the founder effect by haplotype analysis and genealogy. We calculated the combined logarithm of the odds (LOD) score taking a penetrance of 80% into account. The presence of DCM and (p)AF) was further studied in 53 carriers of 16 other TTNtv.Results: TTN c.59926+1G>A: Haplotype analysis revealed a 4 Mb shared region, indicating a common ancestor. Genealogy going back five to nine generations revealed three pairs of common ancestors. Segregation analysis yielded a combined LOD score of 3.6. DCM was the predominant clinical finding (67%: 20/30; mean age of onset 49±14 years, 14 males) in TTN c.59926+1G>A. Paroxysmal atrial flutter or (p)AF was observed in 60% of these DCM patients (12/20). In three patients (p)AF preceded DCM. In the carriers without DCM, three subjects (30%; 3/10) had p(AF) without atrial dilatation. Other TTNtv: Of 53 carriers of 16 other TTNtv, 19 were asymptomatic (36%), six only had (p)AF (11%), in three (p)AF preceded DCM (6%), in 11 (21%) (p)AF was diagnosed in the same year as DCM , six developed (p)AF after diagnosis of DCM (11%) and eight only had DCM (15%).Conclusion: The TTN c.59926+1G>A variant is pathogenic and the first reported TTN founder mutation associated with DCM and a high rate of atrial arrhythmias (20%) even before DCM becomes apparent. This high rate was confirmed in carriers of other TTNtv (17%) and suggests that TTNtv predisposes to atrial arrhythmias. Holter monitoring, in addition to ECG and echocardiography, could be considered in the follow-up of asymptomatic carriers.