Sub-mitral aneurysms are rarely reported clinical entity. Though a variety of etiologies are proposed, congenital weakness of the mitral valve annulus is the most widely accepted one. A 17-year-old boy with sub-mitral aneurysm presented with severe mitral regurgitation, which was diagnosed by echocardiography and successfully managed surgically. The aneurysm wall was positive for Staphylococcus aureus, and patient was treated with intensive antibiotics.