Thyroglobulin level as a predictive factor of tumoral recurrence in differentiated thyroid cancer. Ninety-eight patients with differentiated thyroid carcinoma were studied. Actuarial methods were used to investigate the 10-yr probability of survival (pS) and disease-free survival (pDFS). Our results show that the pDFS is a function of: (1) clinicopathologic stage: Stages I-II, pDFS = 90.9% +/- 5.0% versus Stages III-IV, pDFS = 55.9% +/- 17.8% (p less than 0.005); (2) age: Age less than 45 yr, pDFS = 87.2% +/- 10.0% versus age greater than or equal to 45 yr, pDFS = 66.6% +/- 12.0% (p less than 0.002); and (3) plasma thyroglobulin (Tg) levels: Tg less than or equal to 23 ng/ml, pDFS = 100% versus Tg greater than 23 ng/ml, pDFS = 68.3% +/- 10.6% (p less than 0.005). Using the multivariate analysis of proportional risk, the regression coefficients obtained (Stage: beta = 0.7615; Age: beta = 1.6398, and Tg: beta = 1.7607) allowed us to establish two different groups of risk of relapse on the basis of a prognostic index. 