To assess the value of serum thyroglobulin (Tg) levels in the follow-up of differentiated thyroid carcinoma after ablative therapy simultaneous Tg estimations and radioiodine (131I) scans were performed on patients during an 18-month follow-up period. In this study, 287 scans were performed on 200 patients who were not receiving Thyroxine (T4) replacement at the time, i.e., off T4. Wherever possible, Tg was also estimated while the patient was receiving T4. All sera were screened for Tg autoantibodies which were detected on 67 occasions in 44 patients (22%). Of the 220 sera without Tg autoantibodies (156 patients), 17 were accompanied by scan evidence of functioning thyroid tissue, although Tg was undetectable (less than 5 micrograms/l) either on or off T4. Serum Tg was only detectable off T4 in a further five patients (six scans) who simultaneously had scan evidence of functioning thyroid tissue. In seven patients the finding of detectable Tg preceded scan evidence of recurrence. Thus, serum Tg is useful in the follow-up of differentiated thyroid cancer after ablative therapy. However, some patients with recurrence or metastasis will be missed if Tg alone is relied on, particularly if thyroxine treatment is continued.