The morphological, histochemical, and immunohistochemical findings of seven cases of solid cell nests (SCNs) of the thyroid are described. Light microscopy showed two cell types forming the SCNs, which we refer to as "main cells" and "C cells." In all cases "mixed thyroid follicles" (a unique structure lined by follicular epithelium and epidermoidlike cells) were observed in which the histochemical study confirmed the presence of intraluminal acid mucins. Adult adipose tissue and cartilage were found in one case and foci of cartilage were observed in another case in association with the SCN. Immunohistochemical studies showed positivity of "main cells" for carcinoembryonic antigen (CEA), high- and low-molecular weight keratins, neurotensin, and somatostatin. "C cells" were positive for calcitonin, calcitonin gene-related peptide (CGRP), and chromogranin. The two cell types in SCNs were consistently negative for thyroglobulin. Neuron-specific enolase (NSE)-positive cells were found in the vicinity of the SCN. The unusual association of adipose tissue and cartilage as well as the results of the extended immunohistochemical study in this series provides further support to the belief that SCNs and "mixed thyroid follicles" represent remnants of the ultimobranchial body and should be considered normal components of the thyroid gland.