The relationship between Alexis Carrel, who received the Nobel Prize in Physiology or Medicine in 1912, and Simon Flexner, director of the Rockefeller Institute for Medical Research, is well documented. However, Carrel's collaboration with Abraham Flexner, a key figure in US medical reform, is less widely recognized, and it seems there are no previous mentions of a connection with William Osler. This historical vignette offers insights into the interactions among these three figures during World War I. Five letters, preserved in the library of the National Academy of Medicine in Paris, are presented in full.