Roentgenographic review of the first 210 cases of Ewing sarcoma (ES) in the Intergroup Ewing Sarcoma study revealed that 37.6% of cases had evidence of diffusely increased intraosseous density or diffuse sclerosis (DS). In these cases the sclerosis was usually mixed with various patterns of lysis and/or combined with a periosteal reaction. A radiograph blinded histologic review of selected biopsies showed an 83% incidence of dead bone compared to 23% in those without DS. Ten percent of the cases with DS had appositional new bone formation on dead bone whereas none of the cases without DS showed such reactions. Pathologic explanation of the roentgenographically identified diffuse sclerosis in ES has not been previously well documented in the medical literature. © 1984 International Skeletal Society.