An unusual thyroid carcinoma is described, showing structural, histochemical and radioimmunologic features of both a follicular and a parafollicular cell carcinoma. Radioimmunoassay revealed high levels of thyroglobulin in the patient's serum and in extracts from metastatic tumor tissue. Immunoreactive thyroglobulin was demonstrated histochemically in tumor cells. On scanning, pulmonary metastases showed uptake of 131I. Somatostatin and neurotensin immunoreactivity was also revealed histochemically in the tumor and a large proportion of the neoplastic cells were argyrophil. Serum calcitonin level was normal and no immunoreactive calcitonin was found in tumor tissue by radioimmunoassay or histochemically. Light microscopy showed cribriform patterns suggestive of follicular carcinoma as well as solid areas reminiscent of medullary carcinoma. Electron microscopy revealed two types of tumor cells. One type had electron dense granules resembling secretory granules characteristic of polypeptide hormone and/or monoamine producing endocrine cells. The other type had no such granules but showed a prominent vesicular rough endoplasmic reticulum similar to that seen in neoplastic follicular cells. The results suggest two alternative possibilities regarding the histogenesis of the tumor. One would be a mixed neoplasm, resulting from a coincidental malignant change in both follicular and parafollicular thyroid cells. The other, more likely alternative would be that the tumor cells are derived from a common stem cell with the potentiality of differentiating into both follicular and parafollicular adult cells. The finding that both thyroglobulin and somatostatin or neurotensin immunoreactivity occurred together in some tumor cells supports the latter possibility and suggests that at least some follicular and parafollicular cells may have a common precursor origin.