DLK1 is an imprinted gene on chromosome 14. Using informative coding single nucleotide polymorphisms, we found DLK1 expression to be monoallelic in normal bone marrow, whereas it was biallelic in 76% of acute myeloid leukemia (AML) overexpressing DLK1 (61% of all AML). Quantitative methylation analysis of 7 cytosine-phosphate-guanosine-rich areas (3 upstream of or within DLK1, the putative intergenic-differentially methylated region and 3 upstream of or within MEG3) revealed a strong association between biallelic DLK1 expression and hypermethylation of a cytosine-phosphate-guanosine-rich region 18 kb upstream of DLK1. Allele-specific methylation analysis of this region revealed the alleles to be differentially methylated in normal bone marrow and monoallelic DLK1 AML, whereas there was increased methylation of both alleles in AML with biallelic expression. Moreover, chromatin immunoprecipitation analysis revealed that CCTC-binding factor binds to this region in monoallelic but not biallelic expression samples. Taken together, our data indicate that an insulator located 18 kb upstream of DLK1 plays an important role in regulating DLK1 imprinting.