SUMMARY Myocardial fiber disarray is a distinctive histopathologic finding seen in asymmetric hypertrophic cardiomyopathy. We studied 14 hearts with aortic atresia and intact interventricular septum, six hearts with pulmonic atresia and intact interventricular septum, eight normal infant hearts matched for age of the study hearts, and one nonadult heart with hypertrophic cardiomyopathy and asymmetric hypertrophy and quantitatively analyzed tissue sections through both ventricles and the septum. Normal hearts had an average overall fiber disarray of 8.7% (range 3.8-17%) of the left ventricle including septum.Hearts with pulmonary atresia had an overall disarray of 70.4% (range 13-97%) of the entire right ventricle, and those with aortic atresia 62.1% (range 26-97%) of the left ventricle. The one infant heart with hypertrophic cardiomyopathy showed 15.5% disorder of the left ventricular free wall, 75% disorder of the septum and 47% overall myocardial fiber disarray. Thus, while quantitative criterion distinguished normal from abnormal hearts, they did not distinguish among the various pathologic states. Although extensive myocardial fiber disarray is not exclusive to, or pathognomonic of, hypertrophic cardiomyopathy, it is a useful finding taken in the context of the overall disease. The sensitivity and specificity of this isolated morphologic observation as an indication of hypertrophic cardiomyopathy may be misleading.MYOCARDIAL fiber disarray was recognized as a striking and characteristic morphologic feature of asymmetric hypertrophic cardiomyopathy when it was originally described by Teare.I Later reports of hearts with hypertrophic cardiomyopathy associated with asymmetric septal hypertrophy also described this striking histologic finding,2' 3 and others have stressed the specificity and, in some cases, pathognomonic quality of the two morphologic features of asymmetric septal hypertrophy and myocardial fiber disarray to the condition.4 6Myocardial fiber disarray is not unique to hypertrophic cardiomyopathy; it can be found, to some degree, in normal hearts7 and more extensively in hearts with conditions such as pulmonary atresia, aortic atresial and in the infundibular portion of some hearts with tetralogy of Fallot.9 As the observation of myocardial fiber disarray, per se, cannot distinguish these various cardiac states, quantitative analysis of myocardial fiber disarray was undertaken to distinguish more clearly the fiber disarray of hypertrophic cardiomyopathy from that in other cardiac states.5 Maron et al.6 applied a quantitative method specifically to two congenital heart lesions that were associated with myocardial fiber disarray as extensive, qualitatively, as that seen in hypertrophic cardiomyopathy.6 They concluded that myocardial fiber disarray was, on the average, 10 times more extensive in hypertrophic cardiomyopathy, and that the histologic finding of ex- 1, 1983. tensive myocardial fiber disarray was highly sensitive and specific for hypertrophic cardiomyopathy and was a characteristic ...