The feasibility of percutaneous transluminal cardioscopy of the left ventricle was examined in 35 patients with or without various heart diseases. A guiding balloon catheter (7 or 9 French) was introduced through the right femoral artery into the left ventricle. The balloon was inflated, and a fiberscope (1.6 or 4.3 French) was advanced to the distal tip of the catheter. The balloon was then manipulated against the portion of the ventricle targeted for examination, and heparinized saline (10 U/ml) at body temperature was infused through the catheter at 5 ml/sec for 3 to 6 sec. The images were recorded on videotape and 16 mm cinefilm. In 4 patients without demonstrable heart disease, the endocardial surface of the left ventricle was brown and the trabeculae became prominent during systole. The chordae connected to the mitral leaflets were white. In 3 of 5 patients with dilated cardiomyopathy, the endocardium was light yellow with thin trabeculae which barely contracted during systole. The endocardium was red or reddish-brown in 3 patients with acute myocarditis. In patients with rheumatic mitral stenosis white patches were scattered on the endocardial surface. In 7 of 8 patients the akinetic or dyskinetic segments representing prior infarctions were white. No complications other than transient ventricular arrhythmias were noted. We conclude that percutaneous fiberscopic imaging with a guiding balloon catheter is feasible and safe, and yields highly detailed images of the endocardium.