Lectin histochemistry and electron microscopy were used to study a series of 13 solid or microacinar medullary carcinomas of the thyroid (MCTs) and compare them with four follicular MCTs and other forms of thyroid cancer. Lectin histochemistry was not found to be of diagnostic value, since the MCT did not display any distinct lectin-binding pattern. This approach demonstrated that all MCTs, irrespective of their histologic appearance, consist of polarized cells, arranged into microfollicles that can be demonstrated readily by electron microscopy. We conclude that all MCTs form follicles, some visible by light microscopy, while others are submicroscopic and apparent only on histochemical staining. Because of the histochemical and ultrastructural similarities between solid MCTs and tumors with a follicular pattern, the latter should not be considered a distinct variant of C-cell neoplasia.