Advancing medical knowledge has not infrequently changed our interpretation of clinical syndromes and diseases. On one hand, well-defined symptom complexes, once thought to represent specific diseases, were found to be caused by more than one pathological process and, conversely, different clinical syndromes were revealed as different phases and manifestations of one and the same nosologic entity. Thus, conditions as different in their clinical manifestations as Hand-Sch\l=u"\ller-Christian disease, Letterer-Siwe disease, and eosinophilic granuloma of From the