The simultaneous occurrence of two distinct neoplasms derived from different cells of origin is a recognized, albeit rare, entity. In the thyroid, such lesions could consist of medullary carcinoma composed of parafollicular C cells and well-differentiated carcinoma showing follicular epithelial cell differentiation. We report a patient whose thyroid contained calcitonin-immunoreactive medullary carcinoma and thyroglobulin-positive papillary carcinoma, clearly separated from each other. The tumors metastasized to regional lymph nodes, where they formed foci of composite medullary and papillary carcinoma, with each component maintaining a distinct immunophenotypic profile. The composite metastases are best regarded as collision tumors, as each primary neoplasm exhibited only one line of differentiation. Given the high incidence of papillary carcinoma, the occurrence of the two tumors may be a coincidence. Alternatively, a common tumorigenic stimulus triggering neoplastic transformation of both parafollicular C cells and follicular epithelial cells is a plausible explanation for such a phenomenon.