Malignant Hyperthermia (MH) is an inherited disorder of skeletal muscle in which an abnormality in the regulation of calcium release from internal stores can result in a fatal hypermetabolic reaction on exposure to general anesthetics. Mutations in the gene encoding the skeletal muscle ryanodine receptor/ calcium release channel (RYR1) have been linked to MHS in 50% of overseas families examined, and at least five additional MH susceptibility loci have since been proposed. Current diagnosis of MH in New Zealand relies on the in vitro contracture testing (IVCT) of excised muscle bundles with caffeine and halothane. The genetic basis of MH in NZ families was investigated, with the goal of developing genetic tests to replace the muscle biopsy test. A search for previously published RYR1 mutations in susceptible members of 33 NZ MH families revealed three RYR1 mutations; Arg163Cys, Gly341Arg, and Gly2434Arg, which co-segregated completely with susceptibility to MH (MHS). None of the 17 published RYR1 mutations were detected in a local MHS Maori family in which several anaesthetic deaths have occurred. This is the largest characterised MH family in the world. An examination of the segregation of a panel of chromosome 19q markers with MHS in over 200 members of this family revealed that MHS was linked to the RYR1-flanking markers. This implicated the involvement of a novel RYR1 defect. The entire 15.3 kb RYR1 coding region was combed for mutations by RT-PCR and automatic sequence analysis. A novel point mutation was detected that changed threonine 4826 to isoleucine in the C-terminal region of the RyR1 protein. This mutation was not found in 220 chromosomes from the normal population, or in 94 members of the family who had been diagnosed MHN (normal). A screen for the mutation in 210 key family members revealed a direct correlation between inheritance of the mutation and highly abnormal muscle contracture results in 36 individuals. 22 MHS individuals lacked the mutation; consequently the false positive rate of the IVCT and the possible segregation of at least one additional MHS gene complicated genetic linkage analysis. These problems were addressed by investigating increasingly stringent models for MH diagnosis. Four additional novel RYR1 mutations were detected in other MHS families investigated by sequence analysis of cDNA and genomic DNA, Arg401Cys, Arg2452Trp, Arg2454His and His4833Tyr. The detection of the Thr4826Ile and His 4833Tyr mutations established the channel domain of the ryanodine receptor as a new MHS domain. Genetic testing for MHS can now be applied with caution to predict MH susceptibility in approximately 40 % of at-risk individual in NZ, thus reducing the number of patients requiring an expensive and invasive surgical procedure.