Measurements of circulating thyroglobulin (hTg) and 131I whole body scan were performed in 101 patients with differentiated thyroid carcinoma who had been subjected to surgical thyroidectomy and 131I ablation of remaining thyroid tissue. All 45 patients with positive scans (i.e. functioning metastases) had elevated hTg concentrations. Of fifty-six patients with negative scans forty-two had undetectable or very low hTg levels and were considered to be free of metastatic thyroid tissue, whereas fourteen showed the presence of non-functioning metastases in the clinical and/or radiological examination. In this group of patients, eleven had elevated serum hTg levels while the other three patients had detectable hTg concentrations within the normal range. These results indicate that serum hTg measurements correlate very well with scan findings and have the added advantage of detecting non-functioning metastases which would not be detected by scanning. We concluded that measurement of serum hTg may be used together with scanning, as the first step in the follow-up of thyroidectomized patients with differentiated thyroid carcinoma.