As a member of the Lewis and Clark Expedition, Charles Floyd was the first United States soldier to die west of the Mississippi River. He was among his companions and developed the sudden onset of abdominal pain and was dead the next day. The most common medical opinion for cause of death has been acute appendicitis. However a new edition of his journal reveals signs and symptoms previously overlooked suggesting a genetically determined metabolic etiology, which is explored in this report to the conclusion that he died from acute intermittent porphyria.