Aberrant methylation of Nr4a3 exon 3 CpG island (CpGi) was initially identified during multistep mouse skin carcinogenesis. Nr4a3 is also known as a critical gene for neuronal development. Thus, we examined the Nr4a3 exon 3 CpGi methylation in mouse brain tissues from 15-day embryos, newborns and 12-week-old adults and found significant increase of its methylation and Nr4a3 expression during mouse brain development after birth. In addition, homologous region in human genome was frequently and aberrantly methylated in neuroblastoma specimens. A quantitative analysis of DNA methylation revealed that hypomethylation of CpG islands on Nr4a3 exon 3, but not on exon 1 was identified in three neuroblastomas compared with matched adrenal glands. Additional analysis for 20 neuroblastoma patients was performed and 8 of 20 showed hypomethylation of the CpGi on Nr4a3 exon 3. The survival rate of those 8 patients was significantly lower compared with those in patients with hypermethylation. Immunohistochemical Nr4a3 expression was generally faint in neuroblastoma tissues compared with normal tissues. Moreover, the MYCN amplified NB9 cell line showed hypomethylation and low expression of Nr4a3, while the non-MYCN amplified NB69 cell line showed hypermethylation and high expression. These results indicate that DNA hypomethylation of the CpGi at Nr4a3 exon 3 is associated with low Nr4a3 expression, and correlates with poor prognosis of neuroblastoma. Since Nr4a3 upregulation associated with the hypermethylation and neuronal differentiation in mice, poor prognosis of neuroblastoma associated with Nr4a3 low expression may be partly explained by dysregulation of its differentiation.