• Multicore disease is a nonprogressive myopathy. To our knowledge, in all previous cases, the clinical course has been benign with no deaths attributed to it. We describe a patient who presented as a floppy baby and remained weaker throughout his life than any other patients previously described. Biopsy findings were characteristic of multicore disease. However, at age 21/2 years, our patient developed congestive heart failure that was easily controlled with digitalis and diuretics. Shortly after cardiac catheterization, the patient developed a high, unexplained fever and died 26 hours later despite aggressive attempts at resuscitation. Therefore, patients with multicores in skeletal muscle may have severe weakness and may also have a predisposition to complications subsequent to anesthesia.