In vitro tests for the diagnosis of malignant hyperthermia susceptibility were performed in 105 patients and 20 normal controls according to the protocol of the European MH Group. Results of dynamic halothane and caffeine tests (in which muscle bundle length is cyclically changed) were compared to those of static tests (constant muscle bundle length). Overall, no significant differences between dynamic and static tests were found in threshold concentrations eliciting contractures. Identical diagnostic results with dynamic and static tests were obtained in 94.3% with halothane and in 88.9% with caffeine. The magnitude of contractures was, however, significantly greater in the dynamic than in the static halothane test for halothane concentrations less than 3%, but this did not influence diagnosis. The results indicate that dynamic tests are not superior to static tests, but several tests should be performed to enhance diagnostic safety.