Two cases of Riedel's thyroiditis demonstrated the difficulties encountered in diagnosis. In the first case, subtotal thyroidectomy was performed due to compressive goiter. The intraoperative macroscopic aspect suggested Riedel's thyroiditis. The pathology report also discussed the diagnosis, despite a major lymphoid component, as dense fibrosis had multilated the thyroid parenchyma. Subsequent clinical course led to the diagnosis of lymphoma. The clinical presentation in the second case was classical with rapid increase in size of a hard thyroid gland. Histology was however atypical showing calcifications and liquid cysts within the fibrous thyroid tissue. The initial signs of Riedel's thyroiditis, a very rare disease, usually suggest neoplasia: ligneous compressive goiter with deep adherences. Histologically, there is invasive fibrous thyroiditis with highly suggestive occlusive phlebitis in certain cases. The origin of the disease remains obscure. The differential diagnosis between a sclerosing form and lymphoma is particularly important for therapeutic management.