The European Organization for Research on Treatment of Cancer (EORTC) Thyroid Cancer Cooperative Group presented a prognostic index in 1979 that included all histologic groups of thyroid carcinomas, and was based on a multivariate analysis of 507 patients with a median follow-up of 40 months. The current report not only uses a multivariate analysis to study the clinical validity and reproducibility of this index on case material consisting of 226 differentiated thyroid carcinomas with a considerably long follow-up (11 years), but also it investigates possible prognostic factors, other than those proposed by the EORTC. Three EORTC variables could be reproduced as important: age at diagnosis, locally advanced disease, and distant metastases. Of the additional histopathologic variables tested, microscopic tumor invasion beyond thyroid capsule (pT4) and marked cellular atypia (MCA) proved to be significant. When the effect of the age-correlated tumor factors pT4 and MCA of differentiated thyroid carcinomas were taken into consideration, age alone lost its prognostic importance as a guide for treatment.