Myotonic dystrophy type 1 (DM1) is associated with both skeletal and cardiac muscle involvement. The aim of the present study was to determine whether familial clustering is observed in the severity of muscle involvement in DM1. We evaluated 51 sibling groups constituting 112 patients with genetically-verified DM1. The siblings were similar to each other in age, cytosine-thymine-guanine (CTG) repeat length, age at disease onset, muscular impairment rating score, and electrocardiographic markers of cardiac conduction disease. After adjusting for the similarities between siblings in age and CTG repeat length, the siblings remained similar to each other in measures of both skeletal and cardiac muscle involvement. These results suggest that factors other than CTG repeat length play a role in the severity and progression of the degenerative skeletal and cardiac muscle disease in DM1.