Twenty specimens of heart with mycotic aneurysms at the aortic root were studied. In ten cases, mycotic aneurysm followed infection of the aortic valve. In one case, it developed following infection of an aortic jet lesion, and in nine patients, the aneurysm was at the seat of a prosthetic aortic valve. In seven of the 11 cases with a natural aortic valve, the valve was either unicuspid or bicuspid. A retrospective evaluation of the data on the clinical records of the 20 patients revealed that infective endocarditis or noncardiac postoperative sepsis was present in 11. The most frequently isolated microorganism was Staphylococcus aureus. Conduction disturbances were found in six patients, all of them with involvement of the atrioventricular node by the aneurysm. Perforation into intracardiac cavities was found in four, two into the right ventricular infundibulum and one each into each atrium. Pericardial tamponade was caused by bleeding from the aneurysm in two cases, and myocardial infarction was a probable consequence of coronary arterial compression by the aneurysm in two cases. Mycotic aneurysms of the aortic root, in spite of their being partially or completely healed of active infection, carry a high risk of the complications enumerated. Among the 20 cases, cultures were positive in 11 and negative in nine. Staphylococcus aureus was cultured from five of the cases.