Solid cell nests (SCN) are found within the thyroid parenchyma on routine clinical pathological examinations, but their histogenesis and clinical significance are still obscure. From November 1987 to May 1991, a total of 3,260 patients underwent thyroid surgery at Ito Hospital and SCN were noted in 42 (1.3%) patients. Serial sections of the specimens from these patients were studied morphologically and immunohistochemically. The location and growth pattern of SCN within the thyroid parenchyma were distinct from those of hyperplastic C-cells. SCN were located in the isthmus lobe in 1 patient and even in the pyramidal lobe in 3 patients. In another patient neoplastic proliferation of SCN was observed. Immunohistochemically, SCN were negative for neuron-specific enolase, chromogranin A, and S-100 protein. These findings strongly suggest that SCN are of endodermal origin and that they may be closely related to mucoepidermoid carcinoma of the thyroid gland.