Some investigators have found an increased incidence of papillary carcinoma (PC) of the thyroid in patients with Hashimoto's (autoimmune) thyroiditis (HT), which raises the possibility that there may be more than an incidental association between these 2 diseases. In this study, we analyzed the pathology of Hashimoto's-associated thyroid carcinomas to see if these tumors showed any distinctive features. The possible significance of solid cellular nodules as preneoplastic lesions in patients with HT was investigated. A review of all the cases of HT during a 16-year period yielded 30 PC and 3 follicular carcinomas (FC). Within the PC there were 7 (23%) follicular variants. Twenty (67%) of the PC showed various degree of intratumoral fibrosis, ranging from thick fibrous septa separating tumor nodules to almost complete obliteration of the tumor by the fibrosis, with only microscopic residual tumor nests. In most of the cases, the desmoplastic response within the tumors was of the fibromatosis-like type with dense hyalinized collagen and bland-appearing spindle cells. All the tumors, independently of the degree of fibrosis, showed the nuclear features of PC. No correlation was found between the degree of fibrosis in the tumors and the thyroid gland outside the tumors. There were tumors with marked fibrosis without fibrosis outside the tumors. Four cases of PC (13%) showed a growth pattern characterized by cystic spaces with thick hyalinized walls and focal papillary hyperplasia lined by flat and cuboidal epithelium, reminiscent of a vascular neoplasm. There were 4 atypical solid microscopic nodules with confluent cellularity; 2 of them associated with a PC and the other 2 with diffuse HT without PC. These nodules were composed of cells with clear nuclei and occasional grooves without nuclear pseudoinclusions. By immunohistochemistry, 2 of 3 nodules showed cytoplasmic reactivity for cytokeratin 19, and 2 of 3 nodules were positive for the RET/PTC (rearranged during transfection, papillary thyroid carcinoma) antibody. In summary, HT-associated PC may frequently display prominent stromal desmoplasia and a pseudovascular pattern, both of which can present diagnostic difficulties if the cytologic features of PC are not recognized because of the marked obliteration of the tumor by the fibrosis. Atypical nodules may represent a precursor lesion of PC in patients with HT.