A case of aneurysm of a sinus of Valsalva, with rupture, in a four year old female child is presented. This is believed to be the youngest patient with rupture as yet reported. Studies on the case undertaken in an attempt to verify the diagnosis in vivo included cardiac catheterization, which yielded results compatible with a stream of arterial blood entering the venous circulation at the level of the right ventricle; arterial oxygen analyses; and oxygen uptake determination. From figures obtained by these procedures the extent of the left-to-right shunt was calculated to be about 18% of the systemic blood flow. An attempt was made to correlate the unusually benign clinical course with the results obtained from these procedures.