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We describe five cases in which the cells of lobular carcinoma in situ seemed to form round, regular lumens similar to the cribriform spaces of ductal carcinoma in situ. After careful inspection, we concluded that the spaces indicate the presence of collagenous spherulosis and that this unusual pattern arises through the confluence of lobular neoplasia and spherulosis. We base this conclusion on three lines of evidence. First, attenuated myoepithelial cells, rather than carcinoma cells, form the spaces in question. Immunohistochemical staining for smoothmuscle actin and keratin 8/18 established the myoepithelial nature of these flattened cells because they express the former protein but lack the latter. These results also differentiate the myoepithelial cells from those of conventional in situ carcinomas, which do not contain smoothmuscle actin but virtually always possess keratin 8/18. Second, the material within the spaces looks like the deposits that characterize collagenous spherulosis. They consist of densely eosinophilic protein or radiating, fibrillar ground substances, and they differ in appearance from the disorganized, flocculent mucin and the cellular debris found in some in situ ductal carcinomas. Finally, the neoplastic cells display a loosely cohesive growth pattern that is more in keeping with the properties of lobular neoplasia than ductal carcinoma. Considered together, these three points should distinguish involvement of collagenous spherulosis by lobular neoplasia from cribriform ductal carcinoma in situ. Pathologists must remain alert to this form of mimicry because classification of in situ carcinoma of the breast carries important clinical implications.