Lymphoma of the thyroid is almost exclusively derived from B cells of mucosa-associated lymphoid tissue (MALT), and frequently co-exist with autoimmune thyroiditis in which most infiltrating cells are of Th1 cell origin. We present here two rare cases of peripheral T-cell lymphoma (PTCL) based on chronic thyroiditis with the phenotype CD3+, CD4+, CD8-, TCR+. Furthermore, lymphoma cells in both cases were CXCR3+, CCR5+ and ST2(L)-, suggesting a Th1 cell origin. Eight of 11 cases of PTCL of the thyroid in the literature, including our cases, were associated with thyroiditis. Except for one tumor of T-cell type, all of the five lymphomas analyzed for CD4 expression were positive for the antigen. Among them, both those examined for chemokine receptors were phenotypically of Th1-cell origin with a background of thyroiditis, suggesting that Th1 activation induced by chronic inflammation could lead to PTCL of themselves as well as MALT-lymphoma of B cells.