Objectives: We performed an immunohistochemical study to examine the place of neovascularization in the tumorigenic process of follicular thyroid carcinoma and to determine whether tumor angiogenic activity in follicular carcinoma plays a role in tumor aggression. Materials & Methods: We studied 63 follicular thyroid carcinomas and compared with 22 follicular adenomas. The areas of capsular invasion, vascular invasion and cellular atypism of the tumor were confimed on H & E stains. The paraffin embedded tissues were stained by the use of monoclonal antibodies against Ag CD34. Microvesseles were counted in the area of highest vascular density at 200 times magnification. The microvessel densities(MVD) were analized in relation to histologic type and location of the tumors. Results: There were 59 minimal invasive types and 4 widely invasive types of carcinoma. In the histologic specimens of carcinomas, capsular invasion was identified in all the cases, vascular invasion in 46 and cellular atypism in 24. Mean values of the MVDs of the minimal invasive carcinomas, the widely invasive carcinomas and the adenomas were $263.8{pm}69.2,;256.l{pm}49.3;and;241.5{pm}159.4$, respectively and there was no significant difference between each group. In follicular carcinomas, there was a regional difference of the MVDs. The areas of tumor showing cellular atypism and adjacent to or penetrating the capsule, in which represents the tumorigenic process of carcinoma, had a higher rate of vascularization, than other areas of the tumor(p<0.05). However, these features were not noted in the follicular adenomas. Conclusion: Although there was no significant difference of the MVD between follicular carcinomas and adenomas, there was a regional difference of the MVD within the carcinomas and the values were significantly higher in the more malignant areas, as indicated by cellular atypism and capsular invasion. Therefore, tumor angiogenic activity measured by MVD may play a role in tumor aggression in follicular thyroid carcinoma.