Historians of diabetes have long claimed that physicians were aware of two distinct types of diabetes mellitus by the 1880s, and that these were the direct forerunners of type 1, juvenile-onset and type 2, adult-onset diabetes. French physician Étienne Lancereaux (1829–1910), based on autopsy and clinical studies, classified diabetes either as diabète maigre (thin, or more accurately emaciated, diabetes), which he believed to be pancreatic in origin with a poor prognosis, or diabète gras (fat diabetes), which he believed had a much better prognosis and was not pancreatic in origin. Historians citing Lancereaux have claimed that he observed the former to occur in young and the latter in middle-aged and elderly people. We review the papers of Lancereaux to clarify his clinical observations and understanding of diabetes. Lancereaux’s description of diabète maigre bores little resemblance to juvenile diabetes and all of his thin patients were middle-aged or older. On the other hand, his diabète gras is akin to type 2 diabetes and he might well deserve credit for its characterization.