To define the region of 11p15 involved in Beckwith-Wiedemann syndrome (BWS), we have carried out a molecular genetic analysis of six patients with features of BWS and constitutional cytogenetic abnormalities involving chromosome band 11p15. Molecular analysis confirmed the 11p origin of the duplicated material and defined the smallest region of overlap for such duplications, within which a gene involved in BWS must be located. This region encompasses the beta-globin gene complex (HBB) to 11pter. In both of our informative cases, the 11p duplication was found to be of paternal origin. Two BWS associated balanced translocations of 11p15 were studied to localize the breakpoints on 11p15. Somatic cell hybrids, Southern blotting and fluorescent in situ hybridization (FISH) showed that both breakpoints were between D11S12 and the insulin-like growth factor 2 (IGF2) gene. A non-BWS translocation breakpoint was more proximal, between HBB and calcitonin-A (CALCA). Pedigree analysis showed that both BWS associated 11p15 translocations were transmitted by phenotypically normal mothers. The data are compatible with the hypothesis that the BWS gene is imprinted and that the maternally inherited BWS gene is normally suppressed whereas the paternally inherited allele is active. Thus, duplications of paternal origin would lead to increased dosage of the BWS gene. Similarly increased dosage of the BWS gene could account for the findings in maternally inherited 11p15 translocations by altering normal imprinting, so that the translocated maternal allele remains active. This study defines one or more gene loci for BWS on 11p15.5 in the genomic region from D11S12 to IGF2.