A peculiar destructive granulomatous lesion of bone was first recognized and described independently in 1940 by Otani and Ehrlich (18) and Lichtenstein and Jaffe (14). Lesions of similar type had previously been described by Finzi (6) in 1929, Mignon (16) in 1930, and Schairer (21) in 1938. They did not, however, consider the lesion to be a distinct entity and referred to it as myeloma with prevalence of eosinophils, granulation tumor of bone, and osteomyelitis with eosinophilic reaction, respectively. Lichtenstein and Jaffe's denomination, “eosinophilic granuloma of bone,” has been widely accepted. Up to July 1, 1945, 48 acceptable cases were recorded in the literature. To these are added the 5 reported in this paper, making a total of 53 published cases. Etiology The cause of eosinophilic granuloma of bone is as yet unknown. Trauma has been considered significant, but proof of a causative relationship is lacking in most instances. Ziehl-Neelsen stains and guinea-pig inoculation of material from the lesions have failed to demonstrate tubercle bacilli. Other bacteriological examinations have shown no organism to be present with any degree of frequency. At the present time most authors consider the lesion to be of inflammatory type, but a causative organism has not been demonstrated. Sex and Age of Patients Of the 53 cases which are reported in the literature (including the present series), 36 were in males and 7 were in females; in 10 cases the sex was not indicated. The incidence with respect to age at the time when the diagnosis was established by operation or autopsy is shown in Table 1. The youngest patient was a six-month-old boy, the oldest a fifty-eight-year-old man. As indicated in the tabulation, eosinophilic granuloma is more common in the younger age group, 64 per cent of the cases occurring in those less than twenty years of age. Although most common in childhood, the condition is not limited to children and young adults. Location of Lesions In the great majority of instances the lesions are confined to bone. Nevertheless, a few instances (15, 22) of extra-osseous eosinophilic granuloma, of which Case 3 in this paper is an example, have been described. The osseous lesions are usually solitary and involve the bones of the skull or pelvis, the vertebrae, ribs, and long bones. Multiple lesions are, however, not uncommon. The greatest number of lesions in a single case was 25. The lesions were solitary in 36 and multiple in 10 of the 53 recorded cases. In 7 cases the number of bones involved was not stated. The location of 108 lesions in 46 cases adequately reported with respect to this feature is indicated in Table II. Signs and Symptoms Local and systemic signs and symptoms of slight to moderate degree are present in most instances. Occasionally, however, as in 2 of the present series of 5 cases, the lesions remain quiescent, produce no symptoms, and are discovered only at autopsy.