Poorly tolerated solitary ASD in the baby is rare. The main criteria for diagnosis have been drawn from a series of 9 cases. The symptoms are often atypical, especially at first: usually a marked pansystolic murmur, a tricuspid flow murmur, late appearance of a doubling of B2, and a non-specific cardiac outline, except for the cardiac enlargement and the increased pulmonary vascularity. The ECG is rather more helpful, with constant RVH. At catheterisation, there is a major left-to-right shunt, the pressures in the right and left atria are equal, and pulmonary hypertension is rare. It is particularly common to find associated extra-cardiac abnormalities. Under medical treatment the prognosis is good, and early surgical closure is not normally required.