A baby with Wiedemann-Beckwith syndrome (WBS) and her phenotypically normal mother carried the same paracentric inversion, inv(11)(p11.2 15.5), in the short arm of chromosome 11. A fetus, sib of the affected baby, had the same inversion and ultrasound scan showed exomphalos. The maternal grandmother is clinically and cytogenetically normal. The pattern of affection in this family is consistent with the suggestion that WBS can be caused by lack of a maternally imprinted gene at 11p15.5, and that in this family the inversion disrupts that gene.