Serum thyroglobulin (S-Tg) concentrations were measured in 262 patients with differentiated thyroid carcinoma. Analysis of S-Tg with respect to histology showed that the follicular variety of carcinoma had the highest S-Tg values preoperatively and postoperatively when metastases were found. The S-Tg levels during thyroxine treatment accurately predicted the results of 131-I total body scan in all but two cases. In another patient both S-Tg and total body iodine scan were negative at the time when metastases were discovered at X-ray examination. All but two patients with negative scan had S-Tg less than 10 micrograms/l, whereas patients with metastases had values between 11 and greater than 5000 micrograms/l. In patients with residual thyroid bed uptake the S-Tg range was from less than 2 to 84 micrograms/l. The authors conclude that determination of the S-Tg level can replace total body iodine scan in most patients who have undergone ablative therapy for differentiated thyroid carcinoma.