ABSTRACT— We describe the chronology of hepatic histopathologic alterations in 50 cases of fatal exertional heatstroke related to military training. Five patients who died in the field demonstrated the earliest alterations: fatty change (sometimes microvacuolar), amitotic hepatocellular regeneration, accumulation of hemosiderin, and congestion. In addition, the sinusoids contained polymorphonuclear leukocytes, immature erythroid and granulocytic cells, megakaryocytes, and lymphocytes. Fibrin was not evident. The other 45 patients, who survived up to 8 days following hospitalization, also demonstrated these findings. Microvacuolar fatty change was common in this group of patients, and degenerated hepatocytes resembled Councilman bodies. Coagulative degeneration and submassive necrosis were uncommon. Of those surviving over 12 h, more than half had bile stasis (sometimes ductal), frequently associated with acute cholangitis and ductular proliferation. Regeneration and pigmentary alterations have not been recognized as early hepatic findings in heatstroke, nor have intrasinusoidal bone marrow elements, ductal bile stasis, and acute cholangitis been described. Possible pathogenetic factors, other than hyperthermia and sequellae of shock, include obesity, recent illnesses and immunizations, bacterial toxemia, hemolysis, and bone marrow injury.