Marked dissimilarities in the epidemiology of osteosarcoma, Ewing's tumor, and 
rhabdomyosarcoma indicate differences in their origins. A major clue to the 
genesis of Ewing's tumor comes not from defining persons at high risk but from 
the observation that blacks are at unusually low risk. The neoplasm does not 
aggregate in families and is not part of any known syndrome. No environmental 
causes have been identified. By contrast, osteosarcoma may be caused by external 
or internal ionizing radiation, and it aggregated in families with the same 
tumor or with dissimilar tumors and in certain genetic disorders of bone. In man 
and in dogs, the frequency of the neoplasm is related to bone mass and growth. 
Rhabdomyosarcoma of the upper versus the lower limbs seems related to muscle 
mass. Age peaks in the occurrence of the tumor elsewhere vary with the anatomic 
site; head and neck tumors develop in early childhood and urogenital tumors both 
in early years and in adolescence. The sex ratio (male to female) also varies 
with the site affected. Rhabdomyosarcoma aggregates with certain other tumors in 
families and overlaps with osteosarcoma in some of these relationships but is 
distinguished from that tumor by its excessive occurrence in neurofibromatosis.