Mixed medullary-follicular carcinomas (MMFC) of the thyroid are rare tumours showing the morphological and immunochemical properties of both parafollicular and follicular cell lineages. Their recognition is based on a classical WHO definition, although several other patterns have been described in recent years. We investigated 11 cases of MMFC by immunohistochemistry and in situ hybridization (ISH) to analyse the structural features, the immunophenotypic profile and the calcitonin (CT) and thyroglobulin (TG) gene expression of the neoplasm. Histologically, 10 cases had mixed parafollicular and follicular cell populations in the primary tumour and 1 only in the lymph node metastasis. All cases were immunoreactive for CT (in medullary areas) and TG (in follicular areas and also in the solid component of 8/11 cases). These findings were confirmed by ISH analysis. Combined ISH and immunostaining showed that most cases had separate CT and TG gene expression, although rare cells with concurrent CT and TG gene expression were identified in 2 tumours. We conclude that (a) MMFC display heterogeneous morphological patterns and are a special type of thyroid tumour undergoing divergent differentiation; (b) in MMFC, CT and TG genes are generally not simultaneously expressed by the same cell, although dual expression of CT and TG was present in rare neoplastic elements; and (c) the origin of MMFC, whether they are derived from the ultimo-branchial body or result from neoplastic transformation of different cell populations following common oncogenic stimuli, is unclear.