A patient with the Marfan syndrome and echocardiographic and angiocardiographic evidence of hypertrophic cardiomyopathy is presented. Endomyocardial biopsy was performed. Histologic abnormalities of the endomyocardium noted in this patient were considered to be related to the basic generalized connective tissue abnormality, and the patient subsequently developed myocardial disease compatible with hypertrophic non-obstructive cardiomyopathy. We believe that this case emphasizes the possible co-existence of subclinical myocardial disease in patients with the Marfan syndrome.