Context .- Historical research on pathology and laboratory medicine services in World War I has been limited. In the Spanish American War, these efforts were primarily focused on tropical diseases. World War I problems that could be addressed by pathology and laboratory medicine were strikingly different because of the new field of clinical pathology. Geographic differences, changing war tactics, and trench warfare created new issues. Objectives .- To describe the scope of pathology and laboratory medicine services in World War I and the value these services brought to the war effort. Methods .- Available primary and secondary sources related to American Expeditionary Forces' laboratory services were analyzed and contrasted with the British and German approaches. Results .- The United States entered the war in April 1917. Colonel Joseph Siler, MD, a career medical officer, was the director, and Colonel Louis B. Wilson, MD, head of pathology at the Mayo Clinic, was appointed assistant director of the US Army Medical Corps Division of Laboratories and Infectious Disease, based in Dijon, France. During the next year, they organized 300 efficient laboratories to support the American Expeditionary Forces. Autopsies were performed to better understand treatment of battlefield injuries, effects of chemical warfare agents, and the influenza pandemic; autopsies also generated teaching specimens for the US Army Medical Museum. Bacteriology services focused on communicable diseases. Laboratory testing for social diseases was very aggressive. Significant advances in blood transfusion techniques, which allowed brief blood storage, occurred during the war but were not primarily overseen by laboratory services. Conclusions .- Both Siler and Wilson received Distinguished Service Medals. Wilson's vision for military pathology services helped transform American civilian laboratory services in the 1920s.