This short review focuses on two questions commonly asked about 131I therapy of thyroid cancer. The first relates to radiation dosimetry. What percentage of 131I uptake is needed for effective therapy? Studies have shown that 131I uptake and therapeutic outcome may not be closely related. A more powerful relationship, however, occurs by using percent uptake per gram. A rearrangement of the basic dosimetry equation shows that for a specific level of radiation, percent uptake per gram and effective half-life are inversely related. Formulas developed from this relationship are useful in determining whether uptake is satisfactory but require estimates of effective half-life and tumor mass. The second question is whether post-therapy 131I scans are useful. Because the sensitivity of 131I scanning is related directly to the amount of radioactivity administered, imaging after therapy may show previously undetected lesions that not infrequently are distant metastases. Thus, information from post-therapy scans may 1) after therapy, 2) direct new diagnostic studies, 3) lead to formal dosimetry studies when maximal 131I doses are needed to treat distant metastases, and 4) be necessary to obtain when 131I therapy is administered to patients with elevated thyroglobulin and negative diagnostic scans. Should post-therapy scans always be performed? They are easy to perform and are clinically useful but may be inconvenient to patients and costly overall. At this time, however, there is not a simple paradigm for selecting patients who may forego these scans.