In persons afflicted with left or right atrial isomerism the intrathoracic and intra-abdominal organs are symmetrical. In left atrial isomerism the intrathoracic organs located on the right are similar to those on the left: lungs, for example, have two lobes and long bronchi. In right atrial isomerism there are three lobes of the lung and short bronchi also on the left side of the chest. Both malformation syndromes are associated with a complex structural anomaly of the heart. The prognosis continues to be poor. We describe an inherited gene defect found in five children in a Finnish family and causing right atrial isomerism.