Blood-filled cysts of heart valves are rare in adults. These cysts are diverticuli lined by endothelium and filled with blood. They appear to be benign lesions and should be removed if they cause problems. We present the case of a mobile tricuspid valve blood cyst that was incidentally found in a patient evaluated for systolic heart murmur. Systolic murmur was found to originate from a muscular-type ventricular septal defect of no haemodynamic significance. The lack of echocardiographic evidence of tricuspid valvular dysfunction and indication for repair of co-existent ventricular septal defect suggested a benign course and, therefore, we monitored the patient safely by echocardiography.