Objective To investigate the effects of polycyclic aromatic hydrocarbons ( PAHs) on regulatory T ( Treg) cells in newborns.Methods Blood samples were taken from the umbilical cord of sev-en newborn babies.CD4+CD25+T cells were isolated and treated with or without 300 nmol/L of phenan-threne for 72 hours in the presence of 100 IU/ml of IL-2.The expression of forkhead box P3 (Foxp3), sig-nal transducer and activator of transcription 3 (STAT3) and STAT5 were analyzed by 8-color flow cytometry. RT-PCR was performed to detect the expression of DNA ( cytosine-5 )-methyltransferase 1 ( DNMT1 ) , DNMT3a, DNMT3b and IL-4 at mRNA level.Pyrosequencing in combination with bisulphite sequencing was used to evaluate the methylation within the promoter and the Treg-specific demethylated region ( TSDR) of Foxp3 locus.Treg cells were cultured for 7 hours with autologous Tresp at Tresp/Treg ratios of 1 ∶1, 2 ∶1, 4 ∶1 and 8 ∶1 and stimulated with anti-CD3/CD28 beads and IL-2 for the evaluation of the immunosuppres-sive activities of Treg cells.Results (1) PAHs inhibited the expression of Foxp3 and the function of Treg cells collected from newborns.(2) PAHs significantly decreased the expression of STAT5 and Foxp3, but increased the expression of STAT3 (P<0.05).(3)PAHs enhanced the methylation of the promoter and the TSDR within Foxp3 gene.(4) The transcription levels of DNMT1, DNMT3a and DNMT3b in PAHs treated group were significantly higher than those of the control group (P<0.05).(5) More IL-4 was secreted by PAHs treated CD4+CD25+T cells, indicating that IL-4 was negatively correlated with STAT5, but positively correlated with STAT3.Conclusion PAHs decreased the number and inhibited the function of Treg cells in newborns.The possible mechanism might be related to the abnormal expression of STAT3 and STAT5 in-duced by IL-4 as well as the methylation within Foxp3 gene.