We report a family that was referred to our laboratory after a fatal malignant hyperthermia (MH) accident during general anesthesia. Postmortem study of different muscles of the proband pointed retrospectively to the presence of central core disease (CCD). Of the 8 family members investigated by histology and in vitro contracture testing (IVCT) 5 were found to be MH-susceptible. Neurological examination was completely normal. Histologically, these 5 patients showed a highly variable proportion (6-89%) of cores in type 1 fibers on light microscopy. In 3 patients definite central cores were found, in 1 patient multicore disease was diagnosed, and 1 patients presented with a mixed central/paracentral form. Electron microscopy could detect cores in only 4 out of 5 patients. These results demonstrate the difficulty to diagnose central or multicore disease and suggest that mixed forms within the same family may occur. The one histologically dubious patient in this family shows that the most sensitive test for the diagnosis of this myopathy might be the IVCT.