Ewing's sarcoma of bone (ESB) and peripheral neuroepithelioma (PN) are 
frequently considered to be different tumors. Some researchers have suggested 
that PN is morphologically a neuroectodermal Ewing's sarcoma. We sought to 
determine the extent of neuroectodermal features in conventional ESB on direct 
patient material (25 cases) and to compare these tumors with a similar group of 
readily diagnosed PNs (10 cases). Light microscopic, ultrastructural, and 
immunophenotypic parameters were assessed and compared for both groups. The 
avidin-biotin complex method was used. All tumors were antigenically intact 
since all stained for vimentin or at least one marker. Neuroectodermal antigens 
(neuron-specific enolase, Leu-7 [HNK-1], neurofilament 200 kd, and S100) were 
found in nine of 10 cases of PN and in 17 of 25 cases of ESB. In ESB, an 
atypical light microscopic appearance correlated with the presence of 
neuroectodermal features in most cases, but neuroectodermal phenotype was more 
frequent (68%) than morphological evidence of neuroectodermal differentiation 
(36%). These data support the concept that ESB and PN are both peripheral 
primitive neuroectodermal neoplasms, differing only in extent of neuroectodermal 
phenotype and morphological differentiation.