Serum thyroglobulin (Tg) was measured in 52 patients 3 months to 15 years (mean 5.3 years) after thyroidectomy with or without subsequent radioablation for differentiated thyroid carcinoma, before and after the interruption of suppressive thyroxine (T4) replacement therapy for 5 weeks. Whole body scintigraphy was carried out at the end of the T4 withdrawal period. Serum Tg was undetectable (less than 3 micrograms/l) in 38 patients on T4 therapy, in 18 the scintigraphy showed a minimal accumulation in the neck region and in 20 no uptake anywhere after withdrawal of T4. In the former group Tg rose in 10 patients to 4-21 micrograms/l when off T4 which seemed to correspond to the normal tissue left in situ, in the latter group Tg rose only in 2 patients to 5 and 21 micrograms/l, respectively. Two patients out of 14 with detectable Tg on T4 had pulmonary metastases as uncovered by whole body scintigraphy (in one of them Tg rose from 12 micrograms/l on T4 to 1200 micrograms/l off T4) and 6 patients were suspected for having recidual cancer tissue (2 patients had a negative scintigraphy) because the Tg rose (66-215 micrograms/l) over the upper limit of the reference range (less than 50 micrograms/l) after T4 withdrawal. In conclusion, in the follow-up of patients with differentiated thyroid carcinoma no routine scans are needed as long as serum Tg remains undetectable but further examinations are shortly warranted when detectable Tg is obtained during T4 suppression.