Four patients with Multicore Myopathy, a rare morphologically distinct myopathy, are described. Although previously considered to be a non-progressive or only slowly progressive myopathy, progression to significant disability was seen in three of our cases. The association of cardiac disease with Multicore Myopathy has not been previously emphasised. All four patients in this study had a cardiomyopathy, and heart disease was the cause of death in two of the patients. Multicore Myopathy is not always a benign entity. Cardiac involvement, when present, adversely affects prognosis.