Diabetes insipidus and anterior pituitary dysfunction, are familiar central nervous system (CNS) complications of Langerhans cell histiocytosis (LCH) but the pathophysiology and biological behaviour of other forms of CNS involvement in LCH are poorly understood. In an attempt to improve our understanding of these rare complications, we studied 23 patients with LCH in whom neuroradiological abnormalities, with or without neurological dysfunction other than diabetes insipidus, developed during the course of disease. Neuroradiological abnormalities were of three basic types (a) poorly-defined changes in white matter, (b) well-defined changes in white and grey matter and (c) extra-parenchymal "tumoural" masses. There was a profusion of associated neurological signs and symptoms in most cases but some patients were asymptomatic. The neuropathological features were complex but infiltration of the CNS by histiocytes with xanthomatous change, particularly prominent in mass lesions, was common in the 13 cases in which biopsies were done. Patients with lytic lesions of the skull and diabetes insipidus are evidently most at risk of developing these rare manifestations of LCH. Therapeutic questions could not be answered from this study because no standard treatment had been given and outcome varied widely.