The thyroid gland is an endocrine organ composed of stable cells. It is well known that regenerative capacity of the thyroid tissue is minimal. Various degrees of morphologic alterations do occur in chronic lymphocytic thyroiditis (CLT), including Hashimoto's thyroiditis. Eighty-five CLT cases were analyzed for these morphologic alterations. Small, irregular, atrophic or hyperplastic thyroid follicles were seen adjacent to the lymphocytic infiltration. There was nuclear enlargement, loss of nuclear polarity in thyrocytes and intrafollicular thyrocyte proliferation in these follicles. We thought that the morphologic alterations in involved follicles could be due to regenerative hyperplasia with increased proliferative activity and basement membrane abnormalities. To examine this hypothesis we investigated Ki-67 and laminin immunoreactivity in the involved follicles adjacent to lymphocytic infiltration areas. The uninvolved follicles were used as controls. Immunopositivity of Ki-67 in involved follicles was significantly higher than that in uninvolved follicles (2.97% +/- 2.16 versus 0.83% +/- 1.63, P < 0.001). Laminin immunostaining indicated the destruction or irregular distribution of basement membrane in involved follicles. We conclude that the increased cell proliferation activity and basement membrane abnormalities in the follicles with morphologic changes adjacent to CLT occur in conjunction with regenerative hyperplasia.