Solid Cell Nests (SCN) of the thyroid gland were considered as embryonic remnants or as squamous metaplasia of follicular epithelium. In a retrospective study of 1 390 thyroids, SCN were found in 9.7 % of thyroids (16 % men and 8 % women, chi(2) : p <0, 01). The number of SCN rises with increasing number of sections examined. No statistical differences were found with age or thyroid disorders and presence of SCN, excluding the metaplastic theory. SCN contained in 38 % of cases, neuroendocrine cells (calcitonin and/or calcitonin gene related peptide, chromogranin A) and thyroglobulin cells was observed in 78.3 % of SCN. The different cell types of SCN fitted with the ultimobranchial remnant theory. Thus, some rare carcinomas (like mixed medullary and vesicular carcinoma or mucoepidermoid carcinoma) might originate from SCN.