This paper will use original sources to examine Galen's contributions to the doctrine of stroke, or, in ancient terminology, "apoplexy". Following a rough outline of some methodological issues and pre-Galenic concepts of the disease, Galen's defi nition and construction of the clinical symptoms will be presented in detail. These include sudden onset, comatose state, serious impairment of movement, lack of sensation, shallow respiration, low pulse, damage of voice and usually fatal outcome. Galen distinguished "apoplexy" from other diseases such as "karos", "lethargy" and "paralysis". In retrospect, his defi nition includes a number of modern disease categories, such as myocardial infarction and fatal pulmonary embolism. As for the origin of the attack, Galen's fundamental explanation centered on the Hippocratic concept of humoral imbalance, and he distinctly rejected Aristotle's teaching of the involvement of the heart. According to Galen's teaching, stroke resulted either from an infl ux of blood into the brain or from the accumulation of phlegm and black bile in the cerebral ventricles blocking the transmission of the animal spirit. Prophylaxis and treatment included dietetic, pharmacological, and surgical measures and corresponded to the supposed "hidden cause" as well as observable factors. Galen's Greek texts on stroke remained an authoritative source until the early 19th century.