Five cases of spindle cell squamous carcinoma of the thyroid associated with tall cell papillary carcinoma were examined. The unusual spindle cell squamous carcinoma in these cases resembles those previously described in other sites, including the breast and oropharynx. Three patients demonstrated concurrent spindle cell squamous anaplastic carcinoma and tall cell papillary carcinoma at initial diagnosis, whereas two patients developed concurrent tumor morphologies subsequent to initial diagnoses of only papillary carcinoma. The mean age of the patients was 77 yr, with a female:male ratio of 4:1. Thyroglobulin immunoreactivity was demonstrated in the tall cell papillary component of each case but was absent in both the squamous and spindle cell components. Low molecular weight cytokeratin immunoreactivity was strongly and diffusely present in the tall cell papillary and squamous components, whereas the spindle cell areas demonstrated only focal weak to negative reactivity. Only the squamous cell component demonstrated consistent immunoreactivity with high-molecular-weight keratin antibodies. These unusual tumors have not been previously described in the thyroid, and the association of spindle cell squamous anaplastic carcinoma with tall cell papillary carcinoma in five independent cases may indicate a particular relationship.