Post-therapy whole-body I-131 images were compared to 5 mCi pretherapy diagnostic studies in 39 cases of well-differentiated thyroid carcinoma treated with I-131 to evaluate the utility of this procedure for the detection of residual thyroid tissue and functioning metastases. The post-therapy studies were performed immediately before hospital discharge, when the patient's whole-body retained dose had just fallen below 30 mCi. The mean therapeutic dose given was 121 mCi, and the mean interval between administration of the therapy dose and imaging was 2.4 days. In 18 cases (46.2%), the post-therapy images demonstrated either additional findings, such as unsuspected cervical node or pulmonary uptake, or more accurate localization of abnormalities seen on the diagnostic study. In 6 additional cases (15.4%), questionable new findings were noted. Although the precise implications of these additional findings are uncertain at present, they may have a significant effect on future patient management and follow-up. Therefore, the authors recommend that post-therapy imaging be included in the post-therapy evaluation of these patients. In addition, these findings would also suggest reevaluation of the advisability of using 1-2 mCi doses of I-131 for diagnostic whole-body imaging.