Lobular carcinoma in situ (LCIS) of the breast typically is an incidental finding in breast biopsies performed for a variety of reasons. As patients with LCIS have a bilaterally increased risk of developing invasive ductal or lobular breast cancer, the lesion is presently considered an indicator rather than a direct precursor of breast cancer. However, this view is challenged by the finding that LCIS often accompanies invasive lobular carcinomas (ILC) and that the frequency of ILC following LCIS is far higher compared to unselected patients. To further examine the role of LCIS in the development of invasive breast cancer, we analysed a series of patients with pure LCIS, who later developed invasive breast cancer. Mitochondrial D-loop sequencing revealed a clonal relationship of LCIS and a subset of ILC occurring in the same breast between 2 and 10 years later. Apparently, LCIS has a chimeric role in breast cancer development and is not only a risk factor but in some cases a precursor of ILC.