To establish an objective basis for therapeutic decisions and follow-up programs in patients with follicular thyroid cancer, the authors developed a prognostic scoring system. The prognostic impact of nine clinical, histologic, and therapeutic parameters was quantified retrospectively based on a multivariate analysis covering 149 patients. The relative relapse risk in follicular thyroid cancer (RR) was 6.8-fold increased in the presence of a moderate when compared with a high degree of histologic tumor differentiation. The RR rose with increasing age of the patient at time of tumor diagnosis by a factor of 1.8 per 20 years. The RR was reduced by a factor of 4.3 after the performance of a neck dissection and by a factor of 2.3 after percutaneous radiation therapy of the neck. The relative mortality risk in follicular thyroid cancer (RM) rose in the absence of a tumor capsule by a factor of 10, in the presence of a moderate compared with a high degree of histologic tumor differentiation by a factor of 5.9, in the presence of distant metastases by a factor of 3.2, and with increasing age of the patient at the time of tumor diagnosis by a factor of 2.2 per 20 years. From these data prognostic indices denoting the individual risk for tumor relapse (IRR index) and tumor mortality (IMR index) were calculated. The indices categorize patients into low-risk, medium-risk, or high-risk groups with regard to tumor relapse and tumor-related death. Consequently, the IRR and IMR indices contribute to select patients with follicular thyroid cancer who need an aggressive form of treatment and an intensive followup program. The indices may also be used for risk stratification in prospective therapy trials. Cancer 67:1903Cancer 67: -1911Cancer 67: ,1991. Accepted for publication October 15, 1990. thyroid cancer runs a benign course when compared with other malignancies,* it decreases the quality of life or becomes fatal in a substantial percentage of patient^.^ It is our conviction and that of others'*4 that implementation of risk-oriented medical care in thyroid cancer based on objective criteria, as revealed by mathematical assessment of prognosis, will both improve prognosis and minimize unnecessary treatment. The mathematical assessment of individual prognosis in diseases has become possible with the development of multivariate rn~dels.~ These models determine prognostic factors and quantify their impact on prognosis. In addition, the models permit the calculation of prognostic indices. These indices determine the relapse or mortality risk of a patient as a function of the individual constellation of prognostic factors.Multivariate analyses of prognostic factors in thyroid cancer have recently been reviewed.6 Among these studies, 1903