The Jarisch-Herxheimer reaction is a complication that can follow treatment of several infectious diseases. Its most severe form is in louse-borne relapsing fever; in this syndrome the reaction can cause death. Information from studies in Ethiopia during the past eight years is presented, and clinical, physiological, pathological, and immunological features of the reaction are described. Possible causative mechanisms of the reaction are discussed, especially in relation to the role of endotoxin, and an attempt is made to consider this reaction in relation to other endotoxin-associated states.