The authors report the case of a large mycotic right coronary aneurysm detected at echocardiography in a 45 year old patient with AIDS. Although emergency surgery was planned, the patient died of rupture of the aneurysm with cardiogenic shock and sudden pericardial tamponade. This case underlines the diagnostic value of echocardiography, by the transthoracic approach for para-cardiac masses and with the transoesophageal probe for accurate localisation and demonstration of the coronary origin. In this case, the CT scan was less useful than transthoracic echocardiography. Coronary angiography confirmed the strongly suggestive echocardiographic diagnosis and helped decide management strategy. Atheromatous coronary aneurysms may be treated by stenting but mycotic aneurysms require surgical management.