Seventy-two children with differentiated thyroid cancer who were 16 years old or younger at the time of initial treatment were followed for a median time of 13 yr. Initially, 18% had lung metastases, and 74% had palpable lymph nodes. Capsular invasion was found in 67%, and histological lymph node involvement in 90%. The recurrent laryngeal nerve chain and the jugulo-carotid chain were involved with the same frequency (greater than 80%). The anterior superior mediastinum was involved only in patients with involvement of the recurrent laryngeal nerve chain. Forty-three patients had a complete remission after initial treatment. In patients without distant metastases for whom surgery was macroscopically incomplete, relapses occurred 5 times more frequently than in patients whose surgery was complete. Six patients died from thyroid carcinoma at ages ranging from 19-44 yr, 12-33 yr after initial treatment, and 1 died from intercurrent disease. Despite favorable long term survival (90.3% at 20 yr), the standardized mortality ratio was equal to 8.1. This study underlines the need for complete surgical treatment and compulsive follow-up, which should be continued throughout the patient's life, in order to detect and effectively treat relapses of thyroid cancer.