“Effort syndrome” is a comprehensive term applied conveniently to a group of conditions not apparently due to organic disease, characterized primarily by effort intolerance and manifesting breathlessness, palpitation, left chest pain and a subjective feeling of fatigue on even mild exertion. Vegetative lability on emotional excitement, e.g. palpitation, sweating, giddiness, etc., are described by the patient and may be objectively observed. We have found the simplest and most convenient classification of E.S. patients to be into (a) primarily constitutional, and (b) primarily neurotic groups. In the former (Group 1) the patient has had a poor physical endowment since earliest recollection, to which he has responded in a neurotic manner. In this sense there is a psychological aetiology, but the constitutional factor is the basic one. In the primarily neurotic group (Group 2) there is a definite psychopathology and the usual factors determining a neurosis apply: constitutional physical inferiority, if present, colours the whole picture, but is of only secondary importance. The whole problem has been studied in some detail in the E.S. Unit at this hospital, where more than 2,000 cases have been treated since the end of 1939 (Guttmann and Jones, 1940; Wood, 1941; Lewis, 1941; Jones and Lewis, 1941; Jones and Scarisbrick, 1941, 1942 and 1943).