This report from the Canadian survey of thyroid cancer describes 1,074 patients with papillary thyroid cancer and 504 with follicular thyroid cancer followed for four to 24 years. The study groups included more patients with "advanced" disease and fewer with "early" disease than in the general population because these patients were referred to radiotherapy cancer centers, sometimes routinely, but often because referring physicians believed that certain clinical features indicated the need for additional treatment. Although this report is subject to all the problems of retrospective studies, a careful assessment of the pretreatment extent of disease combined with a long follow-up period has allowed an analysis of prognostic factors with considerable confidence. Univariate analysis of 12 possible prognostic factors (excluding treatment) demonstrated that nine of them were of statistical significance: postoperative status, age at diagnosis, extrathyroidal invasion, distant metastases, nodal involvement, differentiation, sex, tumor size, and pathologic type (in descending order of importance). Multivariate analysis was carried out using cause-specific survival rates. Independently important prognostic factors at initial treatment were age at diagnosis, extrathyroidal invasion, and degree of differentiation histologically for papillary cancers; and extrathyroidal invasion, distant metastases, primary tumor size, nodal involvement, age at diagnosis, and postoperative status for follicular cancers. The prognostic factors for tumor recurrence were quite different for the papillary and follicular cancers and ranked differently for the two groups.