Background: DCIS is usually treated with resection followed by 5 years of adjuvant endocrine therapy for hormone receptor (HR) + DCIS. Endocrine therapy is not used in HR- DCIS. Although DCIS is considered a precursor lesion to invasive breast cancer, the different molecular subtypes confer variable clinical outcomes. The host immune response plays a key role in breast cancer progression and response to therapy. However, relative to invasive breast cancer, the immune milieu of DCIS is less understood. This retrospective study compares the clinical outcomes and tumor microenvironment of HR+ and HR- DCIS in order to identify clinical and immunological features in HR- DCIS that may predict an increased risk of recurrence or progression to invasive breast cancer. Methods: A single institution retrospective chart review was performed to identify patients diagnosed with DCIS between 2012 and 2017. A clinico-pathologic data set, as well as the PD-L1 expression of the DCIS and TILs were collected and correlated with various outcomes. Results: Our cohort consisted of 20 cases of HR- DCIS and 50 cases of HR+ DCIS. Overall, 56% were Caucasian, 20% Asian, 18% Hispanic, and 6% African American. Of the HR- patients, 70% were Caucasian, 15% Hispanic, and 15% Asian. Of the 17 HR- patients with available HER2 data, 76% had HER2+, and 24% triple negative (TN) DCIS. 18% of the HR+ patients and 38% of the HR- patients were PD-L1+. 25% of the HR-/HER2+ patients, and 75% of the TN patients were PD-L1+. 6% of the HR+ patients developed recurrent disease, 2 with DCIS and 1 with invasive ductal carcinoma. 20% of the HR- patients had recurrent disease, all of whom were HER2+. Of the HR- patients that recurred, 2 recurred with metastatic disease, 1 with ipsilateral invasive ductal carcinoma, and 1 with DCIS. All 7 patients that recurred had original DCIS pathology showing a high nuclear grade. Our future results at the time of the meeting will expand on this cohort. Conclusions: This retrospective analysis showed that HR- DCIS conferred higher rates of local and distant recurrence. Therefore, there is a need for treatments to reduce the recurrence rates of HR- DCIS. There are ongoing clinical trials for the high risk, HR-/HER2+ DCIS subtype. TN DCIS is also an aggressive phenotype. Given the high rate of PD-L1 positivity we detected in TN DCIS, immune-based therapy may be useful in the adjuvant setting to reduce the risk of recurrence in this cohort of patients.