A retrospective comparison was performed of whole body scans obtained before and after 143 131I treatments in 93 patients with thyroid carcinoma. Pretreatment scans were performed with 74-185 megabecquerel 131I, and posttreatment scans were performed 5-12 days after dosing with 1.1-7.4 GBq. In 38 (27%) treatment cycles, the results of posttreatment and pretreatment scans differed. Only 14 (10%) posttreatment scans detected new locations of metastatic disease. Seventeen posttreatment scans demonstrated metastatic locations that were already known from previous studies but not seen on the pretreatment scan. Among parameters evaluated (including demographic and histological characteristics), only the combination of age at diagnosis less than 45 yr and history of previous 131I therapy contributed to the likelihood of a new finding on posttreatment scan (relative risk, 3.8). Five of the 14 new posttreatment scan findings were subsequently corroborated by other radiographic studies or thyroglobulin elevations, all in patients with extrathyroidal extension of the primary tumor. Seven (5%) posttreatment scans were unable to detect a focus of uptake seen on the corresponding pretreatment scan. In conclusion, posttreatment scans were most likely to reveal clinically important new information in young patients who had previously received 131I therapy. In older patients and those without previous 131I therapy, posttreatment scans rarely yielded new information that would potentially alter the patient's prognosis.