Findings in 13 patients with central core disease are reported and compared with 62 patients from the literature. Patients with central core disease show wide variation in the clinical spectrum of muscle involvement. Some will have no symptoms and be completely normal on examination, many will show mild to moderate weakness, and a small number will be severely affected. Muscle atrophy is a frequent finding, but progression of muscle weakness is rare and occurs only slowly. Musculoskeletal deformities, including kyphoscoliosis, congenital hip dislocation, feet deformities, and joint contractures, are often seen but generally do not alter the natural history of the disease. Surgical treatment may be required for some of these abnormalities. No relationship appears to exist between the degree of muscle weakness and the presence or type of musculoskeletal deformities. Cardiac abnormalities have rarely been reported in association with central core disease. Mitral valve prolapse was noted in 3 of our patients and 2 others had cardiac arrhythmias. Central core disease and susceptibility to malignant hyperthermia was present in all 11 patients we studied (2 patients with central core disease did not have studies for malignant hyperthermia). These 11 patients came from 4 families. All patients with central core disease should be considered at risk for malignant hyperthermia unless in-vitro contracture tests show that the particular patient is free of the trait.