Desmoid tumor, characterized by aggressive growth and frequent recurrence without metastasis after excision, remains to be a poorly understood neoplasm. In seventeen patients with desmoid tumor of the extremities, the relationship between the cytobiological activity and prognosis of desmoid tumor was studied by reviewing clinical material, histologic examination, statistical analysis of Ag-NORs, and morphometry of tumor cell nuclei in comparison with those of fibrosarcoma and soft tissue fibroma. The results showed that the cells of desmoid tumor were highly proliferative with their biological activity bearing resemblance to fibrosarcoma. Thus we have good grounds for grouping desmoid tumors as of low grade malignancy. We believe that initial surgical resection with a wide margin is the key for good prognosis because a lower recurrence rate will result.