Invasive fibrous thyroiditis (Riedel's goitre) is a rare condition of an unknown aetiology. The condition was first described by Riedel in 1883. In 1904 Hashimoto described another condition of invasive fibrous thyroiditis. Since then it has been discussed if Hashimoto's thyroiditis and Riedel's goitre were one disease in different states or two different diseases. Hashimoto's thyroiditis is known to have an autoimmune aetiology and it can be seen together with pernicious anaemia. Co-existence of Riedel's goitre and pernicious anaemia has never been reported. The patient mentioned had both Riedel's goitre and pernicious anaemia, and was first treated with subtotal thyroidectomy, relapsed after one year and consequently received steroids. Such coexistence and the very good effect of steroids could be explained on the basis of Hashimoto's and Riedel's thyroiditis being one condition in different states and with an autoimmune aetiology. It is recommended that patients with Riedel's goitre are postoperatively treated with steroids.