The long-term monitoring of patients with differentiated thyroid carcinoma (DTC) is essential throughout the patient's life after total or near-total thyroidectomy followed by (131)I remnant ablation and thyroid hormone suppression of thyroid-stimulating hormone (TSH). Sensitive surveillance for DTC recurrence and metastases includes radioiodine diagnostic whole-body scanning (DWBS) and measurement of serum thyroglobulin (Tg) after endogenous or exogenous TSH stimulation. Serum Tg levels during thyroid hormone withdrawal (Tg-off) are usually well correlated with the results of DWBS. In general, undetectable Tg levels with negative DWBS (DWBS(-)) suggest complete remission, whereas detectable or elevated Tg concentrations are suggestive of the presence of (131)I uptake in local or distant metastases. However, DTC patients with discordant results of Tg measurement and (131)I WBS have been observed in the follow-up study. Negative (131)I DWBS and a positive Tg test (DWBS(-) Tg(+)) are found in most of these cases. Positive (131)I DWBS and a negative Tg test (DWBS(+) Tg(-)), though of uncommon occurrence, has also been demonstrated in a small but significant number of cases. With this scenario, one should first attempt to uncover a cause for possibly false-negative or false-positive (131)I WBS or serum Tg. Explanations for the discordance are speculative but should be scrutinized when confronted with discrepant data in a given patient.