At a light microscopic level, the separation of C-cell hyperplasia and microscopic medullary carcinoma of the thyroid (MCT) is difficult, and it ultimately rests on the finding of C cells outside of the thyroid follicular basement membranes (FBMs). To date, this has required ultrastructural examination for proper documentation. The assessment of thyroidectomy specimens from patients with multiple endocrine neoplasia, type 2a (MEN2a), a hereditary condition in which there is widespread C-cell hyperplasia (CCH) and multifocal MCT, presented an opportunity to the authors to assess the entire range of C-cell abnormalities. Total thyroidectomy specimens from 17 patients with MEN2a were examined. In addition to hematoxylineosin (H&E) stains, representative tissue sections were labeled for chromogranin A and collagen type IV (CIV), using the avidin-biotin-peroxidase complex (ABC) method. All patients in the study had multifocal C-cell proliferation that was both diffuse and nodular. Fifteen had microscopic MCTs, which were multifocal in eight instances. Three patterns of C-cell proliferation were recognized in CIV immunostains. The first was characterized by complete investment of C-cells by a continuous rim of CIV, corresponding to FBM and confirming an intrafollicular localization; hence, the diagnosis of CCH was made in such cases. The second pattern was distinctive and was typified by defects in the CIV layer; constituent C-cells assumed an extrafollicular location. These images yielded a diagnosis of micro-MCT. The latter findings were also accompanied by focal reduplication of basement membrane that was apparently tumor derived, producing a micronodular or microlobular configuration. The third pattern represented a combination of the first two, with C-cell nodules that were bounded by CIV and clearly situated in an intrafollicular location; however, focal reduplication of basement membranes was also evident in these cases. The biological significance of the third pattern of CIV staining is uncertain, but it may reflect the presence of a preinvasive proliferation of C-cells that is distinct from "usual" CCH in MEN2a.