A neuroectodermal tumor of bone (NTB) is a small round cell tumor with Homer Wright rosettes. The differences in the nuclear profiles, proliferating activities, and biologic behavior between NTB and Ewing's sarcoma of bone (ESB) are still controversial. In this study, 11 cases of NTB and 12 cases of ESB were compared by a nuclear morphometrical approach using an image analyzer. In addition, the proliferative activity was also evaluated between the two groups by an immunohistochemical study using proliferating cell nuclear antigen (PCNA). The nuclei of NTB were found to be significantly more elliptical (Form Ell: NTB = 0.725, ESB = 0.743, P = 0.017) and irregular (Form Ar: NTB = 0.908, ESB = 0.933, P = 0.046) than those of ESB. The maximum diameter of the nuclei in NTB was larger than those of ESB (NTB = 6.92 micron, ESB = 6.33 micron, P = 0.017), however, there were no significant differences in the nuclear area between the two groups. Immunohistochemically, the mean PCNA score of the NTB (10 cases) were significantly higher than those of ESB (7 cases) (NTB = 25.9, ESB = 11.6, P = 0.005). The mitotic activities of NTB (17/10HPF) were also higher than ESB (6/10HPF) (P = 0.0008). There were no significant differences between the two groups in survival (log-rank test: P = 0.324) according to our small series. Our results suggest that these two tumors should be separated because their proliferating activity is different and they can be separated by some nuclear profiles.