The therapeutic value of I-131 ablation therapy following thyroidectomy for thyroid cancer was evaluated in 54 patients in a prospective study of 25 years. Thirteen (24%) patients had follicular, 24 (44%) papillary, 13 (24%) mixed papillary-follicular, two (4%) Hurthle cell and two (4%) had undifferentiated cell type tumor. Twenty-four (44.5%) patients had metastases at the time of I-131 therapy mainly to cervical and mediastinal lymph nodes, and less frequently to bone, brain, lung, and liver. After surgical thyroidectomy, the mean cumulative dose of I-131 required to achieve therapeutic ablation of functioning post-surgical remnants or tumor metastases was 163.4 mCi. The recurrence rate for patients with metastases was 56% and those without metastases was 25%. Ten patients showed recurrence of I-131 accumulating tissue five to 10 years after initial total ablation. The total mean cumulative dose of I-131 administered for both followup diagnostic studies, and initial and follow-up therapy was 245.3 mCi. Seven deaths were attributable to thyroid cancer, five with differentiated and two with anaplastic cell type tumors. Three of the four patients with differentiated cell type tumors had metastases to brain or bone. Their response to therapy was similar to those patients with anaplastic cell type tumors. In contrast, there were no deaths due to thyroid cancer when total ablation was achieved and maintained. After ablation, all patients were maintained on maximum tolerated doses of thyroid extract or thyroxin. No significant complications attributable to the therapeutic doses of I-131 employed in this series were noted.