The pathogenesis of chronic rhinosinusitis (CRS) with nasal polyps(NP) is still poorly understood. To evaluate the role of Foxp3+ T regulatory cells (Tregs) in the pathogenesis and management of NP, we investigated the location and expression of Foxp3 in NP before and after treatment with intranasal steroid. NP specimens were obtained from 14 patients with NP before and after intranasal administration of mometasone (50 microg/day for 4 weeks). Foxp3 was detected by double immunofluorescence stain, quantitative reverse transcriptase polymerase chain reaction (RT-PCR), flow cytometry and western blot. The concentration of interleukin(IL)-10 in supernatants of homogenized tissue was measured by enzyme-linked immunosorbent assay (ELISA). We found that Foxp3 and IL-10 were downregulated in NP compared to the control mucosa (P<0.05). Foxp3 and IL-10 expression were increased significantly after intranasal steroid treatment (P<0.05). And Foxp3 was tightly correlated with IL-10 in NP (P<0.05) after treatment. These data suggest that Foxp3 is downregulated in NP and intranasal steroid attenuates the chronic inflammatory response by enhancing the expression and function of Foxp3 in NP.