BACKGROUND: Therapeutic approaches in pediatric populations are based on adult data because there is a lack of appropriate data for children. Consequently, there are many controversies regarding the proper treatment of pediatric patients. OBJECTIVE: The present study was designed to evaluate patients with differentiated thyroid carcinoma diagnosed before 20 years of age and to determine the factors associated with the response to the initial therapy. METHODS: Sixty-five patients, treated in two tertiary-care referral centers in Rio de Janeiro between 1980 and 2005 were evaluated. Information about clinical presentation and the response to initial treatment was analyzed and patients had their risk stratified in Tumor-Node- Metastasis; Age-Metastasis-Extracapsular-Size; distant Metastasis-Age-Completeness of primary tumor resection-local Invasion-Size and American-Thyroid-Association classification RESULTS: Patients ages ranged from 4 to 20 years (median 14). The mean follow-up was 12,6 years. Lymph node metastasis was found in 61.5% and indicated a poor response to initial therapy, with a significant impact on time for achieving disease free status (p = 0.014 for response to initial therapy and p<0,0001 for disease-free status in follow-up). Distant metastasis was a predictor of a poor response to initial therapy in these patients (p = 0.014). The risk stratification systems we analyzed were useful for high-risk patients because they had a high sensitivity and negative predictive value in determining the response to initial therapy. CONCLUSIONS: Metastases, both lymph nodal and distant, are important predictors of the persistence of disease after initial therapy in children and adolescents with differentiated thyroid cancer.