Serum thyroglobulin (Tg) radioimmunoassay and 131I whole-body scintigraphy were performed in 134 patients with previous total thyroidectomy for differentiated thyroid carcinoma. Distant spread was found in 46 patients (34%), 42 of whom had serum Tg greater than 10 micrograms/l. Accumulation of 131I in metastases was seen in 39 patients. Serum Tg in patients with iodine-accumulating metastases was significantly higher in follicular than in papillary carcinoma. Scintigraphy showed thyroid tissue remnant and no metastases in 60 patients. Serum Tg was elevated in 19 of these patients and normal in 41. The respective mean uptake of 131I in these two groups was 3.8% and 1.15% of the administered dose (p less than 0.002), indicating that ablation of normal thyroid tissue is important to avoid misinterpreting Tg findings. Scintigraphy performed after 131I in therapeutic doses of 4.5 GBq gave no information additional to that in scans after only 40 MBq 131I. The latter dose thus is adequate for depicting iodine-accumulating metastases. For diagnostic purposes, therefore, a 131I dose of 40 MBq is recommended.