Primary thyroid lymphoma is a very rare tumor and its prevalence is approximately 0.1% of all thyroid cancers in Korea. Its clinical experience is limited and not familiar to physicians. Therefore, we performed this study to investigate the clinical characteristics of primary thyroid lymphoma in Koreans. We retrospectively analyzed the medical records of the 44 patients with primary thyroid lymphoma from 1991 to 2006 at four major referral hospitals in Korea. Out of 44 patients, eight patients were male and 36 patients were female and their average age was 57 years. Fifty-six percent of patients had underlying Hashimoto′s thyroiditis. All but two patients had non-Hodgkin′s lymphoma with B cell origin, and eighty percent of patients presented with stage IE or IIE. Twenty-one of 44 patients (48%) had diffuse large B cell lymphoma (DLBCL) and 17 (39%) had MALT lymphoma. Malignancies originating from lymphoid cells were suspected in 10 of eighteen patients with DLBCL (56%) by using fine-needle aspiration cytology (FNAC), and no difference was noted compared with MALT lymphoma (64%). Compared with DLBCL, the patients with MALT lymphoma were more frequently presented with stage I disease (DLBCL vs. MALT lymphoma: 32% vs. 65%, P=0.04). Moreover, in contrast to DLBCL, most of the patients with MALT lymphoma achieved complete response with treatments (DLBCL vs. MALT lymphoma: 53% vs. 94%, P‹0.01). Because of the low diagnostic accuracy of FNAC, thyroid biopsy should be considered for the diagnosis of thyroid lymphomas in suspicious cases. The patients with MALT lymphoma were more frequently presented with early stage diseases and this might lead to the favorable outcome to treatments than those with DLBCL.