Riedel's thyroiditis is a rare condition with an unknown aetiology. The condition was discovered by Riedel in 1883. In 1904, Hashimoto described another condition of invasive fibrous thyroiditis. Since then it has been discussed whether Hashimoto's thyroiditis and Riedel's thyroiditis are one disease in different states or whether they are two different diseases. Hashimoto's thyroiditis is known to have an autoimmune aetiology and can be seen in conjunction with other autoimmune diseases such as pernicious anaemia. The co-existence of Riedel's thyroiditis and pernicious anaemia is reported for the first time in this case story. Our patient was initially treated with a high dose of steroids and today is well on low-dose steroids and without relapse. The co-existence mentioned, the good effect of steroid treatment, the frequent presence of thyroid autoantibodies and lymphoid infiltration of the thyroid gland resembling that of Hashimoto's thyroiditis might indicate an autoimmune aetiology. It may be that the action on fibroblasts of cytokines known to be released by infiltrating lymphocytes constitutes a possible fibrogenic mechanism, but the primary lesion is still unknown.