Malignant hyperthermia (MH) is an inherited, potentially lethal condition in which sustained muscle contracture with attendant hypermetabolism and hyperthermia is triggered in humans, heterozygous for the gene defect, by inhalational anaesthetics and skeletal muscle relaxants, and in pigs, homozygous for the defect, by stress. Because muscle contracture could result from a defective Ca2+ release channel, we have focussed our attention on the linkage of MH to defects in the gene (RYR1) encoding the skeletal muscle Ca2+ release channel. We have cloned and sequenced human RYR1 cDNA and found restriction fragment length polymorphisms (RFLPs) in the human gene. We also localized RYR1 to human chromosome 19q13.1. Studies of the cosegregation of MH with these RFLPs established RYR1/MH linkage on human chromosome 19q13.1 (lod score of 4.2; recombinant fraction 0.0). We then sequenced MH and normal porcine RYR1 cDNAs. Mutation of C1843 to T, leading to substitution of Cys for Arg615, was the sole amino acid change noted between MH and normal animals. Linkage of this mutation to MH was established in a study of 338 informative meioses (lod score of 102; recombinant fraction 0.0). We identified the corresponding mutation in 1 of 35 human MH families studied and found cosegregation of the mutation and MH. The combination of a high lod score with crossing of a species barrier supports the causal nature of this mutation. Future studies are aimed at finding the major human MH mutations and establishing assays for their accurate diagnosis.