During the last few decades morbid conditions in the reticuloendothelial system have more and more aroused the interest not only of pathologists but of clinicians. Several anatomically, and to some extent clinically, characteristic but, as far as can be judged, pathologically different systemic diseases have been described. For want of concise, characteristically etiologic, clinical or anatomic designations, these conditions have been labeled with the names of the research workers who first investigated the different types of disease—Gaucher's disease, Niemann-Pick disease and Schüller-Christian disease. The latest addition to these diseases is that named Letterer-Siwe disease by Abt and Denenholz.1 When judging the character of a morbid condition in the reticuloendothelial system one should not shut one's eyes to the great difficulties encountered. One is apt to yield to the temptation to limit the conception of the disease too strictly, to regard variations as different diseases or, on the contrary, to