New observations support William Osler's rationale for systemic bloodletting.

Bloodletting has been called William Osler's "blind spot," especially with respect to pneumonia. Also puzzling has been his endorsement of bleeding in selected cases of heatstroke. Recent clinical observations on pneumonia and, to a lesser extent, heatstroke lend support to Osler's rationale for bloodletting in these conditions. A 21st-century rationale now exists for all eight indications for systemic bloodletting listed by Osler in the first edition of his textbook, The Principles and Practice of Medicine (1892): treatment of acute heart failure (seven of the eight indications) or rapid lowering of blood pressure (in subarachnoid hemorrhage accompanied by severe hypertension). These observations support the narrative that during Osler's lifetime, bloodletting became more "rational" largely on the basis of cumulative clinical experience.