Genomic imprinting plays a fundamental role in cancer and some hereditary diseases, including Beckwith–Wiedemann syndrome (BWS), a disorder of prenatal overgrowth and predisposition to embryonal malignancies such as Wilms tumor. We have previously shown that the K_VLQT1 gene on chromosomal band 11p15 is imprinted, with expression of the maternal allele, and that the maternal allele is disrupted in rare BWS patients with balanced germ-line chromosomal rearrangements. We now show that an antisense orientation transcript within K_VLQT1, termed LIT1 (long QT intronic transcript 1) is expressed normally from the paternal allele, from which K_VLQT1 transcription is silent, and that in the majority of patients with BWS, LIT1 is abnormally expressed from both the paternal and maternal alleles. Eight of sixteen informative BWS patients (50%) showed biallelic expression, i.e., loss of imprinting (LOI) of LIT1. Similarly, 21 of 36 (58%) BWS patients showed loss of maternal allele-specific methylation of a CpG island upstream of LIT1. Surprisingly, LOI of LIT1 was not linked to LOI of insulin-like growth factor II (IGF2), which was found in 2 of 10 (20%) BWS patients, even though LOI of IGF2 occurs frequently in Wilms and other tumors, and in some patients with BWS. Thus, LOI of LIT1 is the most common genetic alteration in BWS. We propose that 11p15 harbors two imprinted gene domains—a more centromeric domain including K_VLQT1 and p57^KIP2, alterations in which are more common in BWS, and a more telomeric domain including IGF2, alterations in which are more common in cancer.