The local immunologic phenomena in the thyroid gland of 16 patients with chronic lymphocytic thyroiditis (CLTH) were investigated; 5 of these cases were associated with thyroid non-Hodgkin’s lymphomas (NHL). All patients were admitted because of struma, growing slowly in patients with CLTH alone and rapidly in those with associated thyroid NHL. CLTH was confirmed by histologic findings, including the presence of lymphoid follicles or thyroid autoantibodies in the serum in all 16 patients. Immunologic study revealed that all thyroid NHL were of the B cell type. Surface marker study of suspended cells showed that the percentages of E-rosette-forming cells in patient with CLTH and thyroid NHL (38.0 ± 10.0%) and in patients with CLTH alone (45.0± 11.9%) were between those of B cell NHL of lymph node and B cell hyperplasia reported by others. Immunohistochemical studies confirmed the reactive nature of lymphoid follicles. Subset distribution of T and B lymphocytes in patients with CLTH and thyroid NHL markedly contrasted with that in patients with CLTH alone: an increased ration of CD8+ cells (suppressor/cytotoxic cell) to CD4+ cells (helper/inducer cell). There was a marked increase in the number of immunosuppressive acidic-protein(IAP)-containing macrophages in the thyroid lesion and serum IAP level in patients with thyroid NHL. These findings provide evidence of a difference in the local immunologic conditions in CLTH alone compared to CLTH complicated by thyroid NHL.