We retrospectively analyzed 26 patients with thyroid lymphoma (TL). Patients were mostly females, with a median age of 59 yr, presenting a rapidly growing nodular goiter with or without cervical adenopathy, without symptoms related to lymphoma for 81% and hypothyroidism in 61%. A previous history of Hashimoto thyroiditis was observed in 11 patients. Six different subtypes of lymphoma were observed: 13 of 26 (50%) had diffuse large B cell lymphoma, 6 (23%) mucosa- associated lymphoid tissue (MALT) lymphoma, 3 (12%) had follicular lymphoma, 2 (7%) had Hodgkin's disease, 1 (4%) had small lymphocytic lymphoma, and 1 (4%) had Burkitt's lymphoma. Diffuse large B cell lymphoma patients presented a compressive multinodular goiter, cervical adenopathy (66%), disseminated disease (50%), and poor performance status, with a poor prognosis (5-yr survival at 44%) despite a treatment based on a multidrug regimen. MALT lymphoma arose in patients with previous history of Hashimoto disease, was localized in all but 1, and was biologically associated with hypothyroidism and a high level of serum antithyroid antibodies. With total thyroidectomy, prognosis was good (5-yr survival at 100%). We did not find any routine clinical or biological parameters that could predict the evolution from Hashimoto's thyroiditis to MALT lymphoma. In conclusion, we confirmed the histological heterogeneity of TL corresponding to different clinical presentations and different prognoses.