We have reviewed ovarian mature cystic teratomas with features of chronic thyroiditis that were diagnosed at the department of pathology, Seoul National University Hospital during 7 years, 1984-1991. Twenty three case(8%) containing thyroid tissue among 285 ovarian mature teratomas were available for histopathologic examination. Among these, 4 cases(1.4%) showed lymphocytic infiltration with lymphoid follicle formation. These four cases were examined according to the strict histologic criteria of autoimmune thyroiditis and reactive non-autoimmune thyroiditis for the purpose of differentiation of pathogenesis. Two cases were similar to autoimmune in nature and the other two cases simply seemed to reflect reactive features to adjacent stimulating elements. In conclusion, thyroid tissue present in ovarian cystic teratoma may have histologic features of chronic thyroiditis of either autoimmune or non-autoimmune origin.