Anaplastic thyroid carcinomas usually pose no problems in histologic diagnosis because of the obvious invasive growth, high cellularity, and frank anaplasia. Two cases of a variant of anaplastic thyroid carcinoma with peculiar gross and histologic features closely mimicking those of Riedel's thyroiditis are described in this report. The clinical features were no different from those of the usual anaplastic thyroid carcinomas: occurrence in elderly subjects, presentation with rapidly enlarging neck mass associated with compression symptoms, and rapidly fatal outcome. The tumors were infiltrative, hard, fibrotic masses that partly or completely replaced one lobe of the thyroid, and extended to perithyroid tissues. Histologically, they were predominated by acellular fibrous or infarcted tissue with central dystrophic calcification, as well as hypocellular foci comprising mildly atypical spindle cells intermingled with collagen and small lymphocytes. Both cases showed permeation and plugging of the arteries by tumor. Lymph node metastasis was documented in one case. The spindle cells were positive for epithelial membrane antigen in both cases, and cytokeratin in one. The qualifying term "paucicellular variant" accurately describes this uncommon morphologic variant of anaplastic thyroid carcinoma. It is important to recognize this variant so as not to mistaken it for Riedel's thyroiditis, which is a reactive condition with a very favorable prognosis. The distinguishing features are as follows: presence of infarction, atypical cells in at least some areas, atypical spindle cells obliterating large blood vessels, and immunoreactivity for epithelial markers.