Based on evaluation of 59 probands and their families, we previously demonstrated that over 20% of patients with idiopathic dilated cardiomyopathy (DCM) have familial disease. We acquired 36 additional probands and found familial disease in 24.2% of the 95 probands in this expanded consecutively ascertained cohort. The family history, as reported by the patient and relatives, is often an unreliable indicator of familial disease because patients may be unaware of the significance of a family history of sudden death, arrhythmia or stroke. We demonstrate that careful family history with review of medical records identifies more familial cases than merely asking the patient if there is a family history of DCM. However, even such a careful family history does not identify all familial cases. Some familial cases are identified only by echocardiographic investigation of asymptomatic relatives. We found no clinical attributes of probands, other than family history, which predicted familial disease.