This case report describes an occult papillary adenocarcinoma of the thyroid with pulmonary metastases, discovered at autopsy. Since papillary adenocarcinoma of the thyroid usually invades adjacent structures and cervical lymph nodes in a predictable fashion, the finding of distant hematogenous spread, when the tumor is confined to the thyroid gland, is unusual, as is illustrated by a review of the literature. The patient's history included a poorly differentiated squamous cell carcinoma of the lung, which had been resected 15 years prior to death and had been followed by super-voltage radiation. The possible effects of super-voltage radiation on the pathogenesis and behavior of thyroid cancer are discussed.