Objetive:To investigate whether the methylation patterns of the interleukin-4 (IL-4) gene promoter changed and whether environmental factors affected the methylation level of IL-4 gene in the peripheral blood of patients with recurrent aphthous ulcer (RAU).Methods:Totally 20 patients, who were diagnosed with RAU, were recruited from May 2018 to May 2019 in the Department of Stomatology， First Hospital of Shanxi Medical University in the study (RAU group), including 12 females and 8 males, with mean age of 16-35 years. During the same period, 20 healthy volunteers matching the age and gender of the RAU group were selected from the medical personnel of the same hospital as the healty control group, including 11 females and 9 males, with mean age of 15-35 years. Peripheral blood samples of two groups were collected and the methylation levels of the IL-4 promoter were detected by bisulfite sequencing PCR (BSP). The IL-4 promoter methylation level of each sample was analyzed by direct sequencing and the IL-4 mRNA level was detected by real-time quantitative PCR. The data obtained were statistically analyzed.Results:The IL-4 gene promoter fragment contained 10 CPG sites from -1400 to -1625 bp. The methylation rates of CPG -1556, CPG -1483, CPG -1479and 10 CPG sites were significantly higher in RAU group ［(32.0±19.9)%, (53.0±13.4)%, (46.0±19.8)% and (39.3±12.4)%］ than in healthy control group ［(20.0±3.2)%, (35.5±12.3)%, (28.0±14.4)% and (32.6±5.8)%］, with statistically significant differences ( P<0.05). The relative expression of IL-4 mRNA in the peripheral blood of RAU patients (1.0±0.1) was significantly lower than that of the healthy control group (1.5±0.2) ( P<0.01). There was a significant negative correlation between the overall methylation rate of IL-4 gene promoter and the relative expression level of IL-4 mRNA in RAU group ( r=-0.494, P<0.05). In the multivariate analysis, smoking, vitamin B12 and folic acid in the RAU group were significantly correlated with the overall methylation rate of the IL-4 gene promoter ( P<0.01). Conclusions:The hypermethylation of IL-4 promoter in RAU patients may be related to the reduction of IL-4 gene transcription. Vitamin B12, folic acid and smoking may affect IL-4 gene methylation in peripheral blood of RAU patients.