Résumé : A 38-year-old man with a Starr-Edwards mitral prosthetic valve presented with a Staphylococcus aureus septicemia. Twenty-four hours later, transthoracic echocardiography did not show obvious vegetations but Doppler examination of the prosthetic valve demonstrated a prolonged half pressure time and an elevated peak transmitral velocity. Seventy-two hours after the first septic event transesophageal echocardiography revealed a large annular pannus floating in the left atrium in systole and protruding in the prosthetic cage during diastole. This case report emphasizes the importance of transesophageal echocardiography in septic patients with prosthetic valves and underlines the possibility of extremely rapid pannus formation in these patients.