Abstract Inflammatory breast cancer (IBC) is a rare and aggressive form of breast cancer with poor prognosis. Multimodal treatment including neo-adjuvant chemotherapy is the treatment of choice for IBC patients, with pathological complete response (pCR) being one of the most important prognostic factors. Tumor infiltrating lymphocytes (TILs) are an independent predictor of response to neo-adjuvant chemotherapy in breast cancer (Denkert C. et al., 2010). In a search for multigene predictors of pCR in IBC using DNA micro-arrays, a 107 gene signature was found that distinguished between responders and non-responders. This signature was enriched for immunity-related genes showing that in IBC, as in non-IBC, response to neo-adjuvant chemotherapy is associated with immunity related processes (Bertucci F. et al., 2014). Standard H&E stained sections of formalin-fixed paraffin-embedded pre-treatment tumor tissue of 77 IBC patients were used to evaluate the presence of TILs according to the Denkert method: mononuclear cell infiltrates were considered to be intratumoral TILs when making direct contact with the tumor cells, while stromal TILs are mononuclear cells being present in the tumor stroma without making direct contact with tumor cells. Intratumoral TILs were observed in 10,4% (8/77) of the patients. Stromal TILs (strTILs ) were present in all tumors: in 35% (27/77) strTILs occupied less than 5% of the tumor stroma, in 62 % (48/77) strTILs occupied between 5 and 50% of the tumor stroma and 2,6% (2/77) were "lymphocyte predominant breast cancers (LPBC)" with strTILS occupying more than 50% of the tumor stroma. One LPBC was HER2 positive, the other one was triple negative. There was a significant association of strTILs with response to neo-adjuvant chemotherapy: pCR was obtained in 13% of the patients with strTILs <5%, in 20% of the patients with strTILs between 5 and 50% and in both patients with LPBC (Chi 2 p=0,018). Tertiary lymphoid structures - lymphoid follicles with germinal centres, known to be associated with better clinical outcomes (Gu-Trantien C. et al., 2013) - were detected in 2.6% of the patients (2/77); both patients had a disease free survival of more than 3 years. This study shows that IBC tumors do not contain more TILs than non-IBC tumors and that, as in non-IBC, there is a positive association of stromal TILs with pCR in IBC. Further validation of these results and further study of lymphocyte subsets in IBC is warranted. Citation Format: Cecile Colpaert, Melike Marsan, Peter Vermeulen, Luc Dirix, Steven Van Laere, Inflammatory Breast Cancer International Consortium. Tumor infiltrating lymphocytes in inflammatory breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-04-20.