The relationship of olfactory neuroblastoma to the Ewing sarcoma family of tumors remains controversial due to its variable histopathology and to conflicting or inconsistent cytogenetic, immunophenotypic, and molecular data. To address this issue, we performed a morphologic, immunohistochemical, and molecular study of 20 olfactory neuroblastomas. Morphologically, the tumors consisted of nests of primitive small, round, blue cells, usually set in a background of neurofibrillary stroma. Immunohistochemical stains revealed strong reactivity for neuroendocrine markers (synaptophysin, chromogranin, neuron-specific enolase) and only focal staining for cytokeratins in two cases. Immunostaining with antibody O13 to the Ewing sarcoma-associated MIC2 antigen was uniformly negative (0 of 17). Amplifiable RNA was extracted from paraffin-embedded tissue blocks of 11 cases, and no evidence of the chimeric EWS/FLI transcript, characteristic of Ewing sarcoma, was found in any case. The EWS gene was not rearranged using Southern blot analysis in one additional case in which high molecular weight DNA was available. These results disagree with the proposed classification of olfactory neuroblastoma in the Ewing sarcoma family of tumors and suggest that therapy developed for the latter tumor group may not be biologically rational for olfactory neuroblastoma.