The aim of this retrospective study was to determine the predictive value of antithyroglobulin antibodies (TgAb) in 238 patients with undetectable thyroglobulin after whole thyroidectomy for differentiated cancer and ablative dose of radioiodine. TgAb titers were elevated in 30 patients with undetectable Tg level after withdrawal of T4 treatment. In 21 patients, initially positive TgAb titers became undetectable without clinical event within 3-84 months after therapy (median 12 months). In 2 cases, TgAb levels became undetectable whereas scintigraphy detected pathologic fixations. In 7 cases, high levels of TgAb titles remained unchanged 5 years after primary diagnosis In 3 of them, clinical course and distant metastases, were associated with an increased TgAb level despite normal Tg values. The investigation confirmed that serum TgAb measurement is one of the most important tools for the follow-up of patients with differentiated thyroid cancer. The presence of metastatic thyroid tissue has to be suspected in positive TgAb patients.