This study sought to evaluate whether myeloid-derived suppressor cells (MDSC) could be affected by chemotherapy and correlate with pathologic complete response (pCR) in breast cancer patients receiving neo-adjuvant chemotherapy. Peripheral blood levels of granulocytic (G-MDSC) and monocytic (M-MDSC) MDSC were measured by flow cytometry prior to cycle 1 and 2 of doxorubicin and cyclophosphamide and 1st and last administration of paclitaxel or paclitaxel/anti-HER2 therapy. Of 24 patients, 11, 6 and 7 patients were triple negative, HER2+ and hormone receptor+, respectively. 45.8% had pCR. Mean M-MDSC% were <1. Mean G-MDSC% and 95% confidence intervals were 0.88 (0.23–1.54), 5.07 (2.45–7.69), 9.32 (4.02–14.61) and 1.97 (0.53–3.41) at draws 1–4. The increase in G-MDSC by draw 3 was significant ( p  < 0.0001) in all breast cancer types. G-MDSC levels at the last draw were numerically lower in patients with pCR (1.15; 95% CI 0.14–2.16) versus patients with no pCR (2.71; 95% CI 0–5.47). There was no significant rise in G-MDSC from draw 1 to 3 in African American patients, and at draw 3 G-MDSC levels were significantly lower in African Americans versus Caucasians ( p  < 0.05). It was concluded that G-MDSC% increased during doxorubicin and cyclophosphamide therapy, but did not significantly differ between patients based on pathologic complete response.