We report 13 cases of a peculiar thyroid tumor of follicular epithelial differentiation with distinctly papillary architecture, oxyphilic cytology, and lymphocytic infiltrates in papillary stalks. The majority of these tumors arose in glands with chronic lymphocytic thyroiditis. The combination of oxyphilic cells and lymphocytic stroma gives these tumors a startling resemblance to papillary cystadenoma lymphomatosum, or "Warthin's tumor" of the salivary gland. Twelve tumors occurred in women (age range: 26-66 years, mean 44 years). Two tumors were in the isthmus, six in the right lobe, and four in the left lobe. The only man, 34 years old, had multiple bilateral tumor nodules. The lesions ranged from 0.3 cm to 3.5 cm in maximum dimension. The largest lesion, in a 58-year-old woman, infiltrated skeletal muscle. Three other patients had lymph node metastases, while the nine remaining tumors were confined to the thyroid. Follow-up in these cases suggests that although the histological appearance of these neoplasms is unusual, they behave as typical papillary carcinomas. The striking lymphocytic infiltration and oxyphilic metaplasia in these tumors as well as the association with chronic lymphocytic thyroiditis suggest that as yet undefined immunological mechanisms may play a role in the pathogenesis of this disorder.