Left ventricular luminal changes were examined by percutaneous fiberoptic angioscopy in 13 patients with dilated cardiomyopathy and in four patients with acute myocarditis. Angioscope-guided endomyocardial biopsy was also performed in six patients with dilated cardiomyopathy and in two with acute myocarditis. A balloon-tipped guiding catheter (9F) was introduced through the right femoral artery into the left ventricle, the balloon was inflated, and a 1.6 or 4.3F fiberscope was introduced through the catheter into the ventricle so as to locate the fiberscope tip at the tip of the catheter shaft. The balloon was then pushed against the desired portion of the ventricle and warmed saline was infused to observe the luminal changes. In contrast to the patients without organic heart disease whose left ventricular luminal surface was brown in color, the luminal surface was white or light yellow in four, light brown in one, bluish-white in one, with white and brown portions distributed in a mosaic pattern in four, and it was reddish brown in the remaining one patient with dilated cardiomyopathy. Mural thrombi were observed in two of the patients. The luminal surface was light brown in one, reddish brown in one, rose in one, and red in one patient with acute myocarditis. Thrombi and scattered bleeding were observed in two and one of these patients, respectively. The changes in luminal coloration in patients with dilated cardiomyopathy and acute myocarditis had no obvious relation to left ventricular volume and ejection fraction. Angioscope-guided biopsy revealed that the white and light yellow portions were due to endocardial fibrosis, that the endocardia of brown portions were not fibrotic, and that the myocardium in the red portions contained mononuclear cells, indicating inflammation. The results indicate that the angioscopic features of the left ventricular luminal surface were not uniform in patients with dilated cardiomyopathy or in those with acute myocarditis, and that angioscopy can be used as a guiding tool for endomyocardial biopsy.