Thirteen cases of tall-cell variant of papillary thyroid carcinoma (PTC) showing extensive lymphocyte and plasma cell infiltration within the fibrous stalks of the papillary architecture were compared with age-, sex-, and tumor size-matched cases of ordinary tall-cell variant without extensive lymphocyte infiltration and also with cases of PTC of the conventional type. All cases of the tall-cell variant of PTC with extensive lymphocyte infiltration exhibited the histologic features of chronic thyroiditis. Dissemination of tumor cells with the thyroid was significantly less frequent than in control cases, but there was no difference between the incidences of lymph node metastasis. Immunohistochemically, the lymphocytes infiltrating the carcinoma focus were mainly CD20+, CD45+, and CD45RO+ cells, that is, basically the same as those found in foci of chronic thyroiditis. No tumor recurrence was observed during the mean follow-up period of 3 years 8 months. The results of this study suggest that the tall-cell variant of PTC with extensive lymphocyte infiltration has less aggressive characteristics and a more favorable prognosis.