A 16-year-old boy was referred to our emergency department after falling off his motorcycle. He had no past medical history. On examination, he had a right sided hemiplegia and grade 4/6 systolic murmur. Brain MRI showed a left middle cerebral artery infarction (fi gure A). At echocardiography, a vegetation attached to a fl ail anterior mitral leafl et, and severe mitral regurgitation were seen (fi gure B), suggestive of ruptured chordae tendineae caused by infective endocarditis. A 1 cm diameter ring-form cystic lesion was also seen on the anterior leafl et of the mitral valve (fi gure B). Blood cultures grew Streptococcus viridans. We diagnosed our patient with an embolic stroke due to infective endocarditis. Our patient had a mitral valve replacement 4 weeks after presentation. During the operation, the cystic mass and vegetation were removed (fi gure C and D); histological examination showed a blood cyst and infective endocarditis. Cardiac blood cysts are congenital; they are usually asymptomatic, but can predispose to regurgitation, infective endocarditis, and cerebral emboli. However, they usually regress spontaneously and rarely persist into adolescence or adulthood.