PLCIS (pleomorphic lobular carcinoma in situ) is a recently characterized entity of in situ breast cancer with morphology similar to that of high-grade DCIS (ductal carcinoma in situ) lesions. Features of PLCIS include dyscohesive cells with high grade, atypical nuclei, prominent nucleoli, and moderate to abundant eosinophilic cytoplasm, making it difficult to distinguish from high grade DCIS microscopically. However, PLCIS is a lobular lesion and thus will have negative E-cadherin staining as opposed to DCIS. In this study biopsies diagnosed as high grade DCIS were examined for potential misdiagnosed cases of PLCIS. Examining the 346 biopsies, those with histological criteria compatible with PLCIS were selected. The 20 biopsies satisfying these criteria were stained with E-cadherin to test for the lobular nature of the lesion. A panel of immunohistochemical stains, including HER-2, estrogen receptor, progesterone receptor and Ki-67, was also applied to these cases to establish an immunohistochemical profile of PLCIS. 1.44 % of cases diagnosed as high grade DCIS at Oslo University Hospital, Ullevål in the period 1995 to 2006 were rediagnosed as PLCIS. E-cadherin-negativity is the only identified immunohistochemical stain effective for the histological diagnosis of PLCIS. Ki-67, HER-2, PR, and ER are applied to biopsies for staging of all entities of breast carcinoma. This study pointed at PLCIS to have high expression of PR and ER, but low immunohistochemical positivity on Ki67 and HER2. Classical LCIS is regarded as a non-obligate precursor lesion and is treated with close observation, while DCIS is a precursor to invasive ductal carcinoma and thus prompts removal of the lesion in addition to hormonal- and radiation-therapy. Specific guidelines for treatment of PLCIS have yet to be established. Currently the treatment is the same as for high grade DCIS, but proper data on the actual cancer risk is lacking. PLCIS is still regarded as a non-obligate precursor to invasive breast cancer. More studies must be performed to determine the nature of PLCIS and its potential for invasiveness in order to establish more evidence based guidelines for treatment.