The ultimobranchial thyroid solid cell nests (SCN), irregular structures of about 1 mm in maximal diameter, are usually found in the middle third of the thyroid lateral lobes. SCN are basically composed of non-keratinizing epidermoid cells which lack intercellular bridges and are immunohistochemically positive for a panel of high and low molecular weight keratin proteins, as well as for carcinoembryonic antigen. In addition, SCN display isolated or grouped peripheral calcitonin-immunoreactive 'clear' (C) cells in up to 54 per cent of cases. The SCN central lumen, when present, is usually surrounded by mucinous cells; in addition, it may contain desquamated cells, cell debris, acid mucosubstances, characteristic PAS-positive granular material after diastase treatment, and colloid-like material. The so-called mixed follicles, structures lined by epidermoid cells of SCN and follicular epithelium, are often found as an additional component of the ultimobranchial remnants. The relationship of SCN to thyroid parenchymal cells and the probable implications of the thyroid 'ultimobranchial system' to tumour histogenesis are analysed. Pitfalls that may emerge with regard to SCN in practical pathological approaches are emphasized.