Several aspects of the management of differentiated thyroid cancer cause considerable controversy. Among these is the role of 131I therapy in patients after thyroidectomy. There is no controlled study to demonstrate whether this treatment reduces the recurrence rate or improves mortality. Because of the overall excellent prognosis, it is unlikely that a controlled study will ever be conducted. Most frequently, patients have a diagnostic scan with 131I to determine whether radioiodine would be an appropriate therapy and to judge much to be prescribed, based on the extent of abnormalities seen on the scintiscan. Serum thyroglobulin (Tg) has been found to be a valuable tumor marker, with very good sensitivity and specificity. In most patients, the result of whole-body 131I scintiscan and Tg measurement give concordant results. However, in some patients, Tg is measurable, but the diagnostic scan with 131 is normal. There has been data published about treatment of these patients with therapeutic doses of 131I. The author questions whether this treatment is appropriate, prompted by seeing and hearing of patients who were treated with therapeutic doses of 131I, but had no abnormal uptake of the therapeutic doses and who had no improvement in serum Tg level. These patients have no clinical evidence of disease, and the only abnormality is measurable Tg. Since large doses of radioiodine are not without problems, a controlled clinical trial should be developed to evaluate efficacy in this situation.