We report an unusual case of Gram-negative mediastinitis following aortic valve replacement via median sternotomy. The patient presented two months after surgery following a urinary tract infection in septic shock with a discharging sternal wound and blood cultures positive for Proteus mirabilis. Imaging revealed a large anterior mediastinal abscess and aortic pseudoaneurysm which subsequently ruptured resulting in fatality. Gram-negative mediastinitis is a rare complication of cardiac surgery that can present late following initial clinical improvement and should be considered when "remote site" infections are present. Computerised Tomography scanning has a role to play in the identification of this.