The mitral valve frequently becomes secondarily infected in aortic valve endocarditis. A number of mechanisms have been invoked to explain this association, including: (i) aortic regurgitation (jet lesions); (ii) vegetation prolapse into the left ventricular outflow tract ('kissing lesions'); and (iii) a contiguous spread of infected tissue. A variety of secondary lesions can develop on the mitral valve, including partial thickness ulcerations, perforations, vegetations, and aneurysms. The clinical recognition and treatment of mitral valve aneurysms in primary aortic valve endocarditis are of particular interest and comprise the main focus of this review. Echocardiographically, mitral valve aneurysms appear as narrow-necked, saccular echolucencies protruding into the left atrium. These structures must be distinguished from mitral valve prolapse, congenital diverticula and blood cysts. Mitral regurgitation can result from aneurysm perforation or from the mass effect produced by lesions critically located along the lines of leaflet closure. Aneurysms harboring clot are a potential source of systemic embolization. Treatment is generally surgical; however, otherwise uncomplicated aneurysms have been safely followed using a more conservative strategy.