We report here four additional cases of hyalinizing trabecular adenoma of the thyroid and describe the findings of the accumulated basement membrane (BM) material. Focal lumpy depositions of BM material were noted in the stroma of all four tumors and were particularly prominent in two. Antibodies against type IV collagen and laminin strongly immunoreacted with this material, which also showed diastase-resistant periodic acid-Schiff positivity. The ultrastructural findings of accumulated basement membrane was examined in detail. The most striking ultrastructural findings were seen at the periphery of the cell clusters, where the basal aspect of the tumor cells rested on a highly accumulated BM materials displaying mesh-like, fir-branching, or frame-like appearances. Intracytoplasmic BM islands were frequently observed in neoplastic cells. These BM islands were membrane-bounded and focally continuous with the cell surface membrane, suggesting pseudointracytoplasmic depositions of BM material due to marked cytoplasmic infoldings. From the findings, electron microscopic features of accumulated BM material are very characteristic in hyalinizing trabecular adenomas and can be discernible from other histologic types of BM-producing tumors.