This study examines the influence of tumor capsular invasion on the biological behavior of papillary (PTC) and follicular thyroid carcinoma (FTC) and the prognosis of surgically treated patients. This retrospective cohort study included 350 cases of PTC or FTC from a university teaching hospital. Patient charts were randomly selected and reviewed. The study population was divided into PTC and FTC groups. Each group was subdivided into CI+ (with tumor capsular invasion) and CI- subgroups (without tumor capsule or without capsular invasion). The long-term prognosis was assessed using the American Joint Committee on Cancer pTNM staging and the prognostic index was elaborated by the European Organization for Research and Treatment of Cancer. There were 284 women and 66 men (ages 19-89 years, mean of 44) with an incidence of 53.1 per cent for CI+ tumors. There were no significant differences between the PTC subgroups regarding the short-term clinical outcome and the long-term prognosis. Although patients with CI+ FTC showed lower incidence of lymph node metastasis than patients with CI- FTC, the FTC subgroups were comparable regarding the short-term clinical outcome and the long-term prognosis. Our results suggest that presence of tumor capsular invasion does not adversely influence biological behavior or survival of PTC or FTC. Moreover, the presence of tumor capsular invasion appears to not have significance for the long-term prognosis of patients with PTC or FTC.