Background: Although outcomes from ductal carcinoma in situ (DCIS) are generally excellent, hormone receptor (HR) negative DCIS is associated with a higher rate of recurrence. There is no chemopreventative agent that has been effective in this subgroup. The aim of our study was to evaluate expression of PD-L1 and IDO (Indoleamine 2,3-Dioxygenase) in HR negative DCIS, in an effort to identify a possible target for prevention using immunotherapy. Methods: Using pathology databases at the University of Iowa, we identified 60 cases of HR negative DCIS from 1995-2017, and 41 of these cases had adequate DCIS tissue for immunostaining. Immunohistochemical staining with antibodies against PD-L1 and IDO was performed on slides prepared from formalin-fixed paraffin-embedded tissue blocks retrieved from pathology archives. IDO was assessed by IHC using the mouse monoclonal antibody clone 10.1 (Millipore Sigma). An initial assessment for PD-L1 (M3653 mouse monoclonal antibody clone 22C3, DAKO) was negative. A repeat assessment for PD-L1 on a smaller sample in a different laboratory was also negative. Clinical data was abstracted from the medical record of each patient. Results: Of the 41 subjects with enough tissue for staining, the median age at diagnosis was 56.76 years, and median BMI was 26.59 kg/m². Most common method of DCIS detection was screening mammogram (80.5%) versus a patient detected breast finding (19.5%). Most subjects had high grade DCIS (92.7%) and none had low grade DCIS. Six subjects (16.7%) had an ipsilateral in situ recurrence while another 6 subjects developed ipsilateral invasive cancer. Most of the subjects had HER2+ DCIS (63.9%). None of the 41 cases stained positive for IDO or PD-L1, thus no statistical analysis was done to analyze associations with clinicopathologic characteristics. Conclusions: Our sample of HR negative DCIS tumors did not express PD-L1 or IDO, making these receptors unlikely targets for immunopreventative efforts. It is still possible that tumor mutation burden, expression of PD-L2, PD-L1 or IDO expression on TILs, or expression of CTLA-4 may be future considerations for work in the DCIS microenvironment.