Intranuclear helioid bodies were identified by light microscopy in eight cases of mammary intraductal hyperplasia, only one of which was atypical. These structures appear as round, intranuclear eosinophilic bodies on light microscopic examination. When prominent, they may be and have been mistaken for viral inclusions. Ultrastructural analysis of these inclusions shows a single-membrane-bound structure containing a laminated or homogeneously electron-dense core with a corona of radiating filaments. In the first and only previous report of this structure in the breast, helioid bodies were identified incidentally during an ultrastructural analysis of a breast adenoma, but the light microscopic counterpart of this structure was not described or illustrated. In view of the similarity of the laminated inclusions to psammoma bodies, the possibility that helioid bodies serve as a nidus for development of microcalcification in the breast was considered. Actual microcalcification was not evident in the vicinity of any of these inclusions, however, and there was no evidence of calcification in these structures on von Kossa's stain for calcium.