A miniaturized technique for transcatheter endomyocardial biopsy has been developed in the belief that myocardial biopsy performed in infancy, when the disease process in cardiomyopathy may be most active, should yield important etiologic and nosologic information. To obtain six biopsy specimens, three from each ventricle, adds about 1 hour to a diagnostic right and left heart catheterization. A no. 4 or 5 French forceps with a modified soft shaft is guided to the site in theapical septum of the right and left ventricles through a previously molded to measure guide tube of ultrathin radiopaque Teflon. With biplane fluoroscopy the guide tube of ultrathin radiopaque Telflon. With biplane fluorsocopy the guide tube is introduced as a sheath over a matching catheter and the catheter is removed. Contrast medium in injected to verify position, the forceps is introduced and the biopsy specimen is taken. If the forceps is sharp and pressure on the endocardium in light, evidence of biopsy is not discrenible on examination of the heart 1 week later. The method was developed in small dogs and proved safe and effective in rabbits weighing 3 kg. Biopsy has been performed safely in children aged 4 1/2 months to 5 1/2 years and weighing 4.5 to to 19.6 kg.