Sixteen aneurysms of the septum primum were found at autopsy among 1,578 adults. Most occurred in individuals in their fifth or sixth decade. All aneurysms were asymptomatic and none were diagnosed before death. Fourteen bulged into the right ventricle. The aneurysm may develop in patients with a raised intra-atrial pressure but most seemed associated with normal intra-atrial pressures. A particular morphology of the fossa ovalis region may predispose the septum to aneurysm formation. Echocardiographic and angiographic features of the aneurysm need definition to distinguish them from those caused by other lesions.