Malignant hyperthermia (MH) is an autosomal dominant disorder presenting under general anaesthesia. It is occasionally associated with a myopathy, central core disease (CCD), named after its predominant histochemical characteristic. The penetration of CCD is variable, but typically affected individuals show delayed motor milestones in infancy and remain physically compromised. It was thought until recently that individuals with CCD were always susceptible to MH. Individuals from eight CCD families were screened for the presence of 13 mutations in the skeletal muscle ryanodine receptor gene, reported previously to be associated with MH and/or CCD: none was detected. In seven of these families, where CCD and MH co-existed, we examined the segregation of CCD, MH susceptibility and chromosome 19q markers. In four families, there was complete co-segregation between MH, CCD and the chromosome 19 markers, but in one large pedigree there was a clear lack of segregation of CCD with either MH or chromosome 19 markers and there was no segregation between MH and these markers. This is unequivocal evidence that CCD, in common with MH, is genetically heterogeneous. In the two other families, CCD segregated with chromosome 19 markers but not all individuals with CCD were susceptible to MH. We recommend determination of MH susceptibility in all patients with CCD, irrespective of the MH status of their relatives with CCD.