The implications of the collapse of a soldier early in an exercise from exertional heat illness (EHI) are considered. Such soldiers may be at risk from a genetic predisposition. Malignant hyperthermia (MH) and isolated and improbable cases of EHI may be just two different expressions of the same mutated gene sequence. The genetics of MH are complex and present knowledge is incomplete. The use of the in vitro contraction test (IVCT) on cases of EHI, in addition to its proven role in MH, would be helpful in examining the relationship between MH and EHI. It has been shown that some soldiers collapsing with EHI may have subsequent positive IVCTs. The test, however, sometimes produces false positives and, in addition, a positive result could be a consequence of a heat insult rather than an antecedent. Further studies to establish the incidence of positive IVCTs in relatives of EHI probands, and thus test for heritability, are required. There is, at the moment, only one example of proven MH and proven EHI occurring in the same individual. DNA from a 12-year-old boy who suffered MH and later died from the EHI and from his relatives showed relevant mutations as did the DNA of two of three soldiers who survived EHI. Hajj pilgrims, who collapse with heat illness, do not show such mutations, but the etiology is different. The sedentary pilgrims succumbed to a very high external ambient temperature, the active soldiers to a huge output of internal metabolic heat. Only eventual advances in defining the genetics of MH and EHI will resolve the present confusion of the relationship between the two conditions. Meanwhile, there is a need to bypass considerations of the etiology of EHI and to identify the vulnerable and handicapped soldier by exposure after an interval of time to one or more exercise tolerance tests.