BACKGROUND--Most non-chromosomal congenital heart defects are thought to be caused by the interaction of genetic factors involving multiple genes and environmental factors. Families that have several affected members have been reported, however, which suggests that a single autosomal dominant or recessive gene may cause the cardiac defects. A family in which atrioventricular septal defect seemed to be a single gene disorder is reported. OBSERVATIONS--A family in which the mother and her two daughters from different fathers had atrioventricular septal defect not associated with trisomy 21 is reported. CONCLUSIONS--This family raises the possibility that cytoplasmic or mitochondrial inheritance may be involved in the causation of atrioventricular septal defects. The available data from pedigrees from other cases of familial atrioventricular septal defect do not support this genetic mechanism, but suggest that there is a subgroup without trisomy 21 that has a single gene disorder.