['To determine the stunning effect of a tracer dose of 5 mCi iodine-131.', 'We retrospectively analyzed 145 patients who received the first ablative treatment at our service. Patients were divided according to disease status determined upon post-treatment scanning (101 patients with thyroid remnants and 44 with pulmonary metastases) and whole-body scanning before ablation (performed on 69 individuals). All patients with thyroid remnants were treated with an ablative dose of 100 mCi and those with metastases received 200 mCi.', 'In patients with remnants only (n= 41) or metastases (n= 28) submitted to diagnostic scanning, uptake was found to be apparently increased in most patients cases (71 and 73%, respectively) 7 days after therapy, while reduced uptake (visual) was not observed in any patient. The efficacy of ablation was similar in the groups submitted or not to diagnostic scanning: 71 and 80% in patients without metastases (p= 0.28), respectively, and 43 and 50% in those with pulmonary involvement (p= 0.64).'] Does 5 mCi pretreatment scanning cause stunning when the ablative dose is administered within 72 hours?