AbstractWe report 2 families, each having multiple sibs with abdominal wall defects. In family 1, normal parents gave birth to identical (monochorionic, diamniotic) twins. This is the first reported case of gastroschisis occurring in monozygotic twins. In family 2, a normal mother gave birth to a son with omphalocele. Two subsequent pregnancies with a different husband resulted in a stillborn girl with partial atresia of the colon and a liveborn girl with gastroschisis. In neither case were there any associated anomalies. In neither of the 2 families was there consanguinity or history of other abdominal wall defects. The familial occurrence of these defects suggests that 1) multifactorial determination should be considered in at least some cases of abdominal wall defects, 2) the bowel atresias that occasionally accompany gastroschisis may also have a genetic (multifactorial) cause, and 3) some cases of gastroschisis and omphalocele may have the same genetic cause.