Benua, Leeper, and others (BEL) have advocated the estimation of radiation exposure to the blood to select a more rational maximum safe dose of radioiodine (dosimetry) to treat metastatic functioning well-differentiated thyroid carcinoma. After adopting the BEL dosimetry approach, we reviewed the immediate (during hospitalization) and intermediate (from discharge up to 3 mo) side effects after our initial 15 therapies in ten patients. The doses ranged from 51 mCi (1,887 MBq) to 450 mCi (16.65 GBq). Immediate side effects were observed in 12/15 (80%), are described in detail, and were as follows: gastrointestinal 10/15, salivary 9/15, nonsalivary neck pain, swelling, etc. 2/15, pulmonary 0/15. Intermediate side effects were observed in 10/15 (67%), are described in detail, and were as follows: gastrointestinal 0/15, salivary 3/15, nonsalivary neck pain, swelling, etc. 3/15, nasal complaints 2/15, transient bone marrow suppression 9/10, pulmonary 0/15. No patient required blood transfusions or had complications secondary to reduced blood counts. All patient complaints resolved; however, several patients may have reduced baseline blood counts one year after therapy. No other long-term side effect has been noted but the mean follow-up has been only 15 mo. In our opinion, we have not observed any side effect to date which would contraindicate the continued use and evaluation of the BEL dosimetry approach.