Serum thyroglobulin (Tg) measurements in patients with treated differentiated thyroid cancer are usually well correlated with the presence or absence of residual or metastatic thyroid tissue. However, it is not rare to find a patient with detectable serum Tg levels but negative 131I whole-body scan (WBS) and no evidence of disease activity. To clarify the reason for this discrepancy, we decided to perform the WBS after the administration of a therapeutic dose of 131I in 17 consecutive patients in whom serum Tg was detectable while the WBS performed with a 5 mCi tracer dose was negative (12.6% of 135 patients studied with both WBS and serum Tg). The result of this study demonstrated that after this procedure the WBS became positive for significant residual or metastatic areas of radioiodine uptake in all patients but one. Such data indicate that in our patients the presence of circulating Tg is not a false-positive Tg result, but is due to the presence of residual or metastatic tissue that is not detected in the conventional WBS, that can be visualized using therapeutic doses. Preliminary follow-up data indicate that this procedure may also have therapeutic effect, although the relevance of this aspect remains to be established.