Since the clinical, 2-dimensional and Doppler echocardiographic and pathologic findings in infantile Marfan syndrome have not been documented in detail, a study of 9 such infants was performed. The previously reported 64 cases were reviewed and the salient findings in 22 additional cases were discussed. The age at diagnosis in our 9 cases ranged from birth to 12 months (mean 2.7 months). Mitral valve prolapse was demonstrated in all, with mitral regurgitation in 8. Tricuspid valve prolapse was present in 8, with tricuspid regurgitation in 6. Marked aortic root dilatation was present in all, and was progressive. The aortic root assumed a "clover leaf" appearance in the parasternal short-axis view. Aortic regurgitation was documented initially in 1 patient, and developed during follow-up in 4 of 7 infants. Dilation of the pulmonary arterial root and pulmonary regurgitation were found in 3 of 7 infants. Severe heart failure associated with mitral or tricuspid regurgitation was present in 7 of the 9 patients. Four infants died during the first year of life. The salient pathologic features were myxomatous thickening and redundancy of the mitral and tricuspid leaflets, marked elongation of chordae tendineae and prominent dilatation of the aortic and pulmonary roots. Histologically, the collagen and elastic fibers were severely disrupted, disarrayed and fragmented with increased interstitial ground substance. These data document that infantile Marfan syndrome is characterized by clinical and morphologic features that are distinctly different from the classic syndrome seen in adolescents and adults. The aforementioned findings should facilitate early clinical and echocardiographic diagnosis of infantile Marfan syndrome.