In 1924 Moschcowitz reported the case of a patient who died following a brief illness characterized by fever, haernolytic anaemia, a bleeding tendency, and terminal neurological signs. Histological examination revealed widespread hyaline thromboses in the capillaries and terminal arterioles, Baehr et al. (1936) described fur ther cases and suggested that the thrombi were com posed of masses of agglutinated platelets. The ensuing platelet deficiency was held responsible for the haemor rhagic tendency. This theory remains unproved, but no convincing alternative has been proposed. Thrombotic thrombocytopenic purpura is the most widely accepted name for the condition, and originates from Baehr's concept. Changes in the walls of the vessels underlying the thromboses are presumed to predispose to their formation (Gore and Friedman, 1948). Symmers (1952) has added an extensive review to the limited British literature on the subject. His two cases brought the published total to 38, the majority of them American. Not all have been well substantiated. Since then fur ther cases have been described by Gendel et al. (1952), Barondess (1952), Cooper et al. (1952), and Blackman et al. (1952). The following case, which is typical clinically, is re ported because of the special histological features.