Nine cases of esthesioneuroblastoma were studied; 5 males and 4 females, ages ranging from 13 to 75 years (mean 46). All had polypoid masses located in the nasal cavity and/or paranasal sinuses: 8 cases had a surgical resection and 1 a biopsy due to an unresectable tumor. Histologically, all were cellular tumors composed of small round cells growing in sheets or nests with a fibrillary background. Immunohistochemical studies showed positivity for neuron-specific enolase (9/9), S-100 protein (8/9), synaptophysin (7/9), chromogranin (6/9), neurofilaments (6/9), 013 (4/9), GFAP (1/9), cytokeratin (1/9), and negativity for desmina and CD 45. PCNA index showed three tumors with high proliferative activity (30-50%) and low in 6 cases (0-20%). DNA analysis revealed three diploid and six aneuploid tumors. One patient with an unresectable tumor died and the remaining 8 are alive with a mean follow up of 83 months (6 months to 22 years). We emphasize the utility of several antibodies in the diagnosis of esthesioneuroblastoma, the predominance of aneuploid tumors and the possible unfavorable prognostic value of diploid histograms.