Autoimmune disease is characterized by the presence of organ-and disease-specific autoantibodies in patients and first degree relatives; antibody detection may precede disease onset by several years. We investigated potential involvement of organ-specific autoimmunity in dilated cardiomyopathy (DCM). Using indirect immunofluorescence and absorption studies, organ- and disease-specific IgG cardiac antibodies were found in one-third of DCM patients. Antibody status at diagnosis was associated with better exercise capacity; at 1 -year follow-up two-thirds of antibody-positive patients became negative. These findings suggest that antibodies are early markers; their absence in the majority of patients at diagnosis may relate to long-standing pre-clinical DCM. Antibody screening was performed in asymptomatic DCM relatives, 177 from 33 families with >1 affected individual (familial DCM) and 165 from 31 pedigrees with non-familial DCM. Antibodies were detected in 37 (58%) pedigrees and were more common among relatives than in normals (20% vs 3-5% P = 00001). Antibody-positive relatives were younger, had larger left ventricular end-systolic dimension and reduced % fractional shortening compared to antibody-negative relatives. These findings provide evidence for autoimmunity in a subset (58%) including both familial and non-familial DCM; cardiac-specific antibodies may identify relatives at risk of developing DCM.