Ductal or lobular carcinoma in situ (DCIS/LCIS) can rarely arise from sclerosing adenosis (SA). The combination of cytologically malignant cells and the infiltrative growth pattern may make it challenging to distinguish it from an invasive carcinoma. The authors reviewed 50 consecutive cases of CIS involving SA to seek the salient histologic characteristics in order to prevent overdiagnosis. The features commonly seen with CIS were the lobular configuration at low magnification (94%), uninvolved SA in neighboring tissue (84%), at least focally identifiable myoepithelial cells on H&E-stained sections (76%), separate foci of unequivocal CIS (58%), associated microcalcifications (54%), and hyaline basement membrane surrounding tumor cell nests (48%). The group of DCIS with high nuclear grade showed a tendency to occupy the entire lobule of SA, whereas those with non-high grade were more often partially involving the affected lobule. The presence of adjacent separate foci of CIS was more closely related to the DCIS lesions when compared to those of LCIS. The finding of an SA lobule entirely involved by CIS was significantly correlated with the presence of an invasive carcinoma; this should thus strongly prompt the pathologist to search for other evidence of invasion. Awareness of these features is an additional, useful tool for reaching a proper diagnosis.