The usefulness of cross-sectional echocardiography during endomyocardial biopsy was examined in 10 consecutive patients with myocardial disease of undetermined origin. Twenty-eight endomyocardial biopsies were obtained from the left ventricle and 34 from the right ventricle. Echocardiography was performed simultaneously with monitoring of the biopsy procedure by means of fluoroscopy, pressure measurement, and electrocardiographic recording from the bioptome. Echocardiographic visualisation of the biopsy procedure was feasible in 100% of left and in 18% of right ventricular biopsies. Conventional positioning of the bioptome was corrected in a total of five cases because of inappropriate localisation as apparent from cross-sectional echocardiography. In the left ventricle the site of biopsy could be defined more precisely by echocardiography than by fluoroscopy. At the present stage of technical development the most important potential of ultrasonically guided endomyocardial biopsy seems to be the feasibility of obtaining selective biopsies from well defined areas of the left ventricle when serial analysis from a reproducible area is necessary.