Squamous cell carcinoma of the thyroid (SCT) is an unusual neoplasm thought to arise as a primary tumor or as a component of an undifferentiated carcinoma. The role of p53 and Ki-67 as prognostic indicators in this type of tumor is not known. We studied eight cases of primary SCT. Three cases were analyzed for Ki-67 by immunohistochemistry and for p53 by immunohistochemistry and loss of heterozygosity. Seven patients were women, and one was a man (age range, 31 to 90 years). SCT were firm, were tan with areas of necrosis, and ranged in size from 2 to 8 cm. Histologically, they had islands of squamous cells with spindle cell areas (two of eight). In four of eight cases, SCT was associated with the tall cell variant of papillary carcinoma (TCV). Positive staining for p53 was seen in two of three cases, and in one of three the TCV was also positive for p53. Mean MIB1 labeling index was 30% and 17% in SCT and TCV, respectively. At the time of presentation, six of eight patients had cervical lymph node metastases. In one case, the primary tumor had SCT and TCV; however, only the SCT component metastasized. After mean follow-up of 48 months, one patient had died of disease, five were alive with recurrent or metastatic tumor, and two were lost to follow-up. Primary SCT is an aggressive neoplasm that may be found in association with TCV. p53 expression and high MIB1 labeling index occur in these tumors and may be useful prognosticators.