Coexistence of differentiated thyroid cancer (DTC) and thyroid autoimmune diseases could represent a mere coincidence due to the frequent occurrence of autoimmunity, but there may also be a pathological and causative link between the two conditions. The coincidence of DTC with Hashimoto's disease has been variably reported at between 0.5 and 22.5% and of DTC with Graves' disease between 0 and 9.8%. In this review available evidence for thyroid autoimmunity in DTC is summarized and it is concluded that thyroid cancer does coexist with thyroid autoimmunity, implying that patients treated for autoimmune thyroid diseases should have a careful follow-up. Furthermore, the presence of thyroglobulin antibodies (TgAb) in patients with DTC may limit the use of serum thyroglobulin as a tumor marker due to methodological problems in the determination of serum thyroglobulin. However, in such cases serial TgAb measurements may be used as a surrogate marker for recurrence of thyroid cancer during the long-term monitoring of DTC patients.