Aortic valve replacement was complicated by sternal wound infection with Mycobacterium fortuitum. The wound was treated with débridement and antibiotic therapy. Five months later the patient developed fever, and blood cultures yielded M fortuitum. At surgery, aortitis with pseudo-aneurysm formation was encountered. Mycobacterium fortuitum grew from the aortic lesion. This is the first report of M fortuitum causing aortitis, although this organism is known to infect sternal wounds and mediastinum. Mediastinal infection can progress despite a normal wound appearance. Its manifestations may be delayed and may include infections of the aorta as well as of the sternum and mediastinum.