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A pathologic study of mitral valvular lesions among 55 infants revealed 29 cases in which the mitral valve was the site of a primary congenital malformation and 26 cases in which the mitral lesion was an acquired infarction of papillary muscles secondary to some other malformation. Among the 29 cases with primary congenital anomalies of the mitral valve, there were 41 lesions represented. Congenital anomalies were identified according to the four components of the valve as follows: leaflets, commissures, chordae tendineae, and papillary muscles. The most common basis for primary congenital mitral valvular disturbance was an abnormality of the papillary muscles. In this group, parachute mitral valve and abnormal position of papillary muscles associated with endocardial fibroelastosis were most common (eight and 10 examples, respectively). Anomalous mitral arcade and obstructing papillary muscles were observed four and three times, respectively. Involvement of leaflets was the second most common type of congenital anomaly, being observed 11 times. In this group, in order of decreasing frequency, were supra-valvular ring (five cases), accessory mitral valvular tissue (three cases), "Ebstein's" malformation of the left atrioventricular valve in corrected transposition (two cases), and cleft mitral valve (one case). Commissural fusion was observed once and, in two cases, involvement of multiple components of the valve was observed. Among the 26 examples of infarction of papillary muscles, exclusive of 10 cases with endocardial fibroelastosis, the fundamental congenital anomalies included aortic stenosis (15 cases), coarctation of the aorta (six cases), and anomalous origin of the left coronary artery from the pulmonary trunk (five cases).