In tricuspid atresia, an obligatory right to left shunt occurs at the atrial level. We have observed several patients with left to right interatrial shunts. Data from cardiac catheterisation in 40 consecutive patients were reviewed to determine the frequency and mechanism of left to right shunting in tricuspid atresia. An increase of 6% or more in oxygen saturation between the superior vena cava and the right atrium in two or more sets of saturations, representing a left to right shunt, was present in 29 out of 50 (58%) catheterisations in which the data were adequate. In most, the shunt was also seen cineangiographically in the laevophase. In only two catheterisations was an anatomical cause (ostium primum atrial septal defect in one and anomalous pulmonary venous return in the other) found. In the remaining 27 catheterisations, no anatomical cause was found. Age, Qp:Qs, and mean atrial pressure difference were similar between the shunt and non-shunt groups. In the shunt group right atrial "a" waves were equal to or higher than left atrial "a" waves and left atrial "v" waves were equal to or higher than right atrial "v" waves. Simultaneous pressure recordings (in one patient with left to right atrial shunt) from the left atrium and right atrium with isosensitised miniature pressure transducers mounted 5 cm apart showed (1) a higher pressure in the right atrium than in the left atrium during atrial systole and (2) a higher pressure in the left atrium than in the right atrium during atrial disatole. It is concluded that (a) left to right shunt across the atrial septum occurs frequently in tricuspid atresia and (b) the left to right shunt is the result of instantaneous pressure differences between the atria.