Following surgery which left no microscopic residue, 114 patients with differentiated thyroid cancer (58 papillary, 56 follicular) were subgrouped on the basis of the dose of externally applied prophylactic postoperative radiation: group 1 received an adequate dose of radiation (> or = 4,500 cGy of telecobalt, > or = 4,000 R of orthovoltage therapy), while group 2 received an inadequate dose of radiation (this group included non-irradiated patients). Local/regional-relapse-free survival (LRRFS), distant-metastatic-relapse-free survival (DMRFS) and total-cause-specific survival (TCSS) were calculated by means of life-table analysis for each histologic type. TCSS and LRRFS were significantly (p < 0.001) better for group 1 in papillary cancer. No difference was found in DMRFS. LRRFS was significantly (p < 0.001) better for group 1 in follicular cancer. No differences were found in TCSS and DMRFS. We conclude that (1) prophylactic postoperative external irradiation is an effective method for survival prolongation in papillary cancer, reducing local/regional recurrences, and (2) external irradiation should be considered in the postoperative management of follicular cancer, to diminish local/regional relapse.