A 55-year-old man with the classical mucocutaneous lesions of xanthoma disseminatum has been followed up for a period of 13 years. The special features of this case, which make it unique, are as follows: (1) the availability of histologic data on multiple lesions for more than a ten-year period; (2) the progressive nature of the multiple osseous lesions; (3) the metabolic studies that show no evidence for accumulation of abnormal sterols in a xanthoma, the blood, or intestinal aspirate; (4) the development of hypothyroidism and symptoms or signs, or both, of an intracerebral and an intraspinal lesion; (5) the partial regression of the cutanous symptoms and lesions while receiving clofibrate, in spite of progression of the mucous membrane and osseous lesions, and (6) the failure to develop diabetes insipidus to date.