A European Malignant Hyperpyrexia Group has been formed to facilitate exchange of information between centres performing in vitro muscle testing for malignant hyperpyrexia susceptibility. Data have been collected according to a protocol agreed by the Group. Based on these results, test criteria have been established to allow the following diagnoses to be made: MH susceptible (MHS); MH normal (MHN) or MH equivocal (MHE). It is accepted that MHE classified patients will be under permanent review, pending the collection of further data.