SUMMARYThe case is described of a man who developed attacks of cataplexy, narcolepsy, and sleep paralysis because of a microglioma which infiltrated the walls and floor of the IlIrd ventricle and the upper brain stem. The mechanisms by which the pathology is related to the symptoms are discussed.In 1880 Gdlineau coined the term 'narcolepsy' and related the clinical details of two patients who suffered from the condition, acknowledging that an account of the first had been published by Caffe in 1862. Henneberg (1916) described cataplexy with narcolepsy, and in the 1920s sleep paralysis became associated with the disease. Despite widespread interest and a large literature the cause remains unknown, and the vast majority of cases have no known structural basis. In the minority narcolepsy is said to be 'secondary' to various pathological conditions-to injury (Drake, 1949), to brain stem infarction (Van Bogaert, 1926), to third ventricular tumour (Fulton and Bailey, 1929), to polioencephalitis (Pohl, 1966)-or has occurred with other neurological disease, disseminated sclerosis (Ekbom, 1966;Berg and Hanley, 1963), and cerebellar haemangioblastoma (Tridon et al., 1969). We do not know of a case of primary narcolepsy coming to necropsy, and postmortem details of secondary narcolepsy are distinctly rare (Van Bogaert, 1926;Fulton and Bailey, 1929;Benedek and Juba, 1943; Pohl, 1966). Levin (1932)