Eight mutations in the ryanodine receptor gene on human chromosome 19q13.1 have been linked to malignant hyperthermia. Although 50% of malignant hyperthermia families are not linked to the ryanodine receptor gene, only single malignant hyperthermia families have been linked to alternative loci on chromosome 7q21-22 and on chromosome 3q13.1. Problems in malignant hyperthermia linkage studies may arise from the inadequacy of the caffeine halothane contractures test for phenotypic diagnosis of malignant hyperthermia.