Background: The link between physical activity (PA) and the immune system (IS) is known. However, it is not yet fully understood the immune mechanisms activated by PA. We investigated the immune effect of moderate PA (MPA), nordic or fit walking, during neoadjuvant chemotherapy (NACT) in patients (pts) with breast cancer. Methods: Pts received sequential epirubicin and cyclophosphamide for 4 cycles followed by paclitaxel for 12 weeks. Blood samples from pts underwent MPA (TR) were collected before starting chemotherapy (CT) at baseline (T0), at day 1 of week 6 of paclitaxel (before starting MPA) (T1), before surgery (S) (T2) and after S (T3). Samples were also collected in a group of pts who declined MPA (UN) at the same time points and in 15 healthy volunteers (HV). MPA consisted of 3 workouts per week, 1 hour each, in the 9 weeks before S. At each time point the level of 17 cytokines (IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12, IL-13, IL-15, CCL-2, CCL-4, CXCL-10, CCL-22, IFN-γ, TGF-β, TNF-α, VEGF) was measured. The difference among the median value of cytokines was analyzed using non parametric Mann Whitney U test. Principal component analysis (PCA) was computed to compare the best discriminating cytokine, identified by ROC analysis, of pts at T1, T2, T3 and in HV. Each patient was distributed in the PCA. Pts having similar cytokine values were plotted in the near position. Normalized values of 8 cytokines (IL-2, IL-4, IL-5, IL-6, IL-13, IL-15, CCL-2, VEGF) were used in PCA. Results: Data from 27 pts are available: 10 TR and 17 UN. A significant increase of IFN-γ, IL-5, IL-8, CCL-2 and CXCL-10 between T0 and T1 (P = 0.004, P = 0.013, P = 0.032, P = 0.046, P = 0.046, respectively) was found in the whole population. CXCL-10 significantly increased also between T1 and T2 in UN pts (P = 0.033). TR pts showed a significant lower level of IL-6, IL-13, CCL-2 at T2 (P = 0.012, P = 0.038, P = 0.023) and higher IL-15 level at T3 (P = 0.047) compared to UN pts. Moreover, a significant decrease of IL-5 was observed between T2 and T3 (P = 0.031). PCA showed that TR and UN pts were mixed at T1. HV were clustered all together and distinct from pts. At T2 TR pts moved toward HV and mixed with them while UN remained separated. TR pts tended to separate from HV at T3, while UN pts still remained distinct. Conclusions: NACT upregulated median values of IFN-γ, IL-5, IL-8, CCL-2 and CXCL-10; CXCL-10 value continues to increase during CT only in UN pts supporting the inflammatory effect of CT. On the contrary, during MPA the level of IL-6, IL-13, CCL-2 decreases in TR compared to UN pts. All together these data suggest that MPA damps the inflammatory response to NACT. Our results show that the majority of TR pts reach an immune profile similar to that of HV in PCA. However, at T3 the effect of MPA is dampened, suggesting a potential negative effect of S.