CD4+ regulatory T (Treg) cells are associated with immune tolerance and antitumor immunosuppression. The aim of the present study was to investigate the role and molecular mechanism of C‑C motif chemokine ligand 11 (CCL11) in the regulation of Treg cells from patients with breast cancer (BC) and healthy individuals in vitro, and from tumor‑bearing mice in vivo. CD4+ T cells isolated from patients with BC or healthy individuals were incubated with anti‑CCL11 neutralizing antibodies or recombinant human CCL11 protein, in the presence or absence of a STAT5 inhibitor. The serum CCL11 level and proportion of Treg cells characterized as CD4+ CD25+ forkhead box P3+(Foxp3) among the CD4+ T cells in patients with BC and healthy individuals were analyzed by ELISA and flow cytometry, respectively. CCL11, C‑C motif chemokine receptor 3 (CCR3), Foxp3, phosphorylated‑STAT5 and STAT5 expression levels were determined by western blotting. The serum CCL11 level and the proportion of CD4+ CD25+ Foxp3+ Treg cells were significantly increased in patients with BC compared with healthy individuals. CCL11 blockade reduced the proportion of CD4+ CD25+ Foxp3+ Treg cells, the expression of CCR3 and Foxp3, and the level of STAT5 activation in tumor‑associated CD4+ T cells, in a dose‑dependent manner. CCL11 blockade also reduced the proportion of CD4+ CD25+ Foxp3+ Treg cells and the serum levels of interleukin (IL)‑2 and transforming growth factor (TGF)‑β1 in tumor‑bearing mice. The recombinant human CCL11 protein increased the proportion of CD4+ CD25+ Foxp3+ Treg cells, the expression of CCR3 and Foxp3, and the release of IL‑2 and TGF‑β1 in non‑tumor‑associated CD4+ T cells via the STAT5 signaling pathway. The results of the present study may aid in identifying therapeutics that could further modulate the immune system during BC.