In this study, we examined the hypothesis that dilated cardiomyopathy (DCM) shares genetic risk factors with other diseases of presumed autoimmune etiology, and, therefore, the same multiple genes in combination with environmental factors lead to numerous different autoimmune diseases. In accordance with this hypothesis, we showed an increased prevalence of autoimmune diseases in first-degree relatives of patients with DCM. Also, T-cell activation, as reflected in high levels of the soluble interleukin-2 receptor, appears to identify patients with DCM with a clustering of autoimmune diseases.