Lobular carcinoma in situ (LCIS) is a type of non-invasive breast cancer that is confined to the cells lining the very end of the milk ducts (lobules) in the breast. LCIS has some of the characteristics of cancer cells but does not invade surrounding breast tissue or other structures.

LCIS is usually located in more than one area in the breast (multicentric) and may be present in both breasts (bilateral). It is usually discovered by accident, in the process of obtaining a biopsy of the breast for another problem. It does not typically show up on a mammogram.

Experts disagree about whether LCIS is a preinvasive cancer or a marker for the later development of an invasive cancer. A woman with LCIS has an increased chance of having invasive breast cancer during her lifetime.