To investigate the prevalence, clinical characteristics, and prognosis of familial cases of idiopathic dilated cardiomyopathy (IDC), family screenings were carried out in 117 IDC patients and their relatives. Familial occurrence was suspected in 29 families (25%). Ten families (9%) with 24 patients were confirmed to be familial, but the other 19 families (16%) remained suspected. The age at the time of diagnosis was lower and the cardiac symptoms tended to be milder in the familial group than in the non-familial group, but there were no differences in other clinical parameters. There was also no difference in the survival rate. However, when only NYHA class III and IV patients were selected, the 1-year and 5-year survival rates were lower in the familial group than in the non-familial group. Congestive heart failure was the most common cause of death in the non-familial group, while sudden death was the most common cause of death in the familial group. Among familial IDC patients who were deceased, the left ventricular end-diastolic pressure was higher and the cardiac index was lower at the time of diagnosis than those in patients who were still alive. We conclude that, since the prognosis of familial IDC patients is poor once their cardiac symptoms become severe, early diagnosis and treatment are extremely important.