Perinatal exposure to diethylstilbestrol (DES) induces reproductive tract cancers later in life in both humans and animals. Because there is no clear evidence that perinatal DES exposure induces gene mutation, we proposed that perinatal DES exposure causes epigenetic methylation changes that result in persistent alterations in gene expression, leading to tumorigenesis. The proto-oncogene c-fos is one of the immediately induced genes in uterine epithelium after estrogen simulation and a key player in uterine carcinogenesis. Here, we investigated c-fos expression in mice neonatally exposed to DES (2 microg/pup/day on postnatal days 1-5). The mRNA levels of c-fos in uteri of neonatal DES-treated mice were persistently 1.4-1.9-fold higher than that in the control mice from day 5 to day 60. Overall, the uterine c-fos expression level in the neonatal DES-exposed group was significantly higher than that in the control group. After examination of the methylation status of the c-fos gene, we found that the CpGs in promoter and intron-1 regions were completely unmethylated. In exon-4, from day 17 to day 60, the percentage of unmethylated CpGs was higher in neonatal DES-exposed mice uteri than that in control (42%, 51%, 47%, and 42% in DES-exposed mice vs 33%, 34%, 33%, and 21% in control mice at day 17, 21, 30, and 60, respectively). These results suggest that perinatal DES exposure may permanently alter gene expression and methylation, and the methylation modification may occur in either the promoter regions or other regulatory sites in the gene.