Many patients with hypertrophic cardiomyopathy have signs and symptoms of myocardial ischemia and dysfunction. Although hypertrophy and increased left ventricular pressure can account for such abnormalities, altered small intramural coronary arteries have also been described in such patients. To determine the prevalence and extent as well as the clinical relevance of abnormal intramural coronary arteries, a histologic analysis of left ventricular myocardium obtained at necropsy was performed in 48 patients with hypertrophic cardiomyopathy (but without atherosclerosis of the extramural coronary arteries) and in 68 control patients with either a normal heart or acquired heart disease. In hypertrophic cardiomyopathy, abnormal intramural coronary arteries were characterized by thickening of the vessel wall and a decrease in luminal size. The wall thickening was due to proliferation of medial or intimal components, or both, particularly smooth muscle cells and collagen. Of the 48 patients with hypertrophic cardiomyopathy, 40 (83%) had abnormalities of intramural coronary arteries located in the ventricular septum (33 patients), anterior left ventricular free wall (20 patients) or posterior free wall (9 patients); an average of 3.0 +/- 0.7 abnormal arteries were identified per tissue section. Altered intramural coronary arteries were also significantly more common in tissue sections having considerable myocardial fibrosis (31 [74%] of 42) than in those with no or mild fibrosis (31 [30%] of 102; p less than 0.001). Abnormal intramural coronary arteries were also identified in three of eight infants who died of hypertrophic cardiomyopathy before 1 year of age. In contrast, only rare altered intramural coronary arteries were identified in 6 (9%) of the 68 control patients (0.1 +/- 0.05 abnormal arteries per section; p less than 0.001) and those arteries showed only mild thickening of the wall and minimal luminal narrowing. Moreover, of those patients with abnormal intramural coronary arteries, such vessels were about 20 times more frequent in patients with hypertrophic cardiomyopathy (0.9 +/- 0.2/cm2 myocardium) than in control patients (0.04 +/- 0.02/cm2 myocardium). Hence, abnormal intramural coronary arteries with markedly thickened walls and narrowed lumens are present in increased numbers in most patients with hypertrophic cardiomyopathy studied at necropsy and may represent a congenital component of the underlying cardiomyopathic process.(ABSTRACT TRUNCATED AT 400 WORDS)