Mitral-aortic intervalvular fibrosa aneurysm is a rare disease whose aetiology remains a matter of debate. Here we present the youngest reported patient with the disease, a 6-month-old boy, without a history of infection, which supports a congenital origin as initially proposed. Multidetector-row CT (MDCT) surpassed echocardiography in delineating the intracardiac anatomical details with high spatial resolution, confirming the important problem-solving role of MDCT in the diagnosis of congenital heart disease.