Background: The link between physical activity (PA) and immune system is known. However, are not clearly understood which mechanisms are activated by PA. We investigated the effect of moderate PA (MPA), on a panel of circulating cytokines during neoadjuvant chemotherapy (CT) in patients (pts) with breast cancer. Methods: Pts received sequential epirubicin and cyclophosphamide for 4 cycles followed by weekly paclitaxel ± trastuzumab. MPA consisted of Nordic or fit walking, 3 workouts/week, 1 hour each, in the 9 weeks before Surgery (S). Blood samples were collected in pts underwent MPA (TR) and in a group of pts who declined MPA (UN) at baseline (T0), at day 1 of week 6 of paclitaxel (starting MPA) (T1) and before S (T2). At each time point the level of 22 cytokines (IL1b, IL2, IL4, IL5, IL6, IL7, IL8, IL10, IL12, IL13, IL15, IL17a, IL18, IL21, CCL2, CCL4, CXCL10, CCL22, IFNγ, TGFβ, TNFα, VEGF) was measured using Simple Plex Ella. Sample size was calculated using effect size method considering a Cohen’s d of 0.65 as observed for a candidate cytokine (IL-6) between two groups. Considering a desired statistical power of 0.8 and a probability level of 0.05 a minimum of 78 patients were required. The difference among the median value of cytokines was analysed using non parametric Mann Whitney U test. Comparisons between time points in the same group were analysed with Wilcoxon signed-rank test. Results: Accrual is completed. 61 pts were TR and 20 UN. Main patients’ characteristics are listed. At T0 IL4, IL17, IL18 and VEGF level was significantly higher in UN respect to TR pts. At T1 IL4, IL17 and IL18 level remained significantly higher in UN pts. At T2, UN pts showed significantly higher value of IL4, IL6, IL7, IL8, IL17, IL18 VEGF and TGFβ. IL21 was significantly higher in TR pts. Longitudinal analysis between T0 and T1 in TR and UN pts showed significantly lower level of CCL22 and significantly higher level of IL7, IL13 and CXCL10 in TR pts and significantly higher level of IL8 in UN pts. Longitudinal analysis between T1 and T2 showed significantly higher level of IL21, CXCL10, CCL22, TNFα and significantly lower level of IFNγ, IL6, IL8, CCL2, TGFβ in TR pts and significantly lower level of CXCL10 in UN pts. Conclusions: TR and UN pts differed modestly at baseline including acceptance of MPA. The longitudinal analysis T0-T1 showed that CT induced more evident changes in TR than in UN pts. Longitudinal analysis T1-T2 demonstrated major changes in Th1 and Th2 cytokines only in TR pts. Overall, CT induced weak changes in UN and greater changes in TR pts strengthened by the addition of MPA. These observations suggest a more efficient immune system in TR pts and a possible additional benefit derived from MPA.