With a focus directed on the potential diagnostic yield of tissue biopsy in suspected myocardial disease, transvascular endomyocardial biopsies were carried out prospectively in 130 patients. In 149 out of 153 consecutive procedures, where the number of associated major complications was less than 1%, King's bioptome and a specially designed radiopaque long introducer catheter were used. The rather thickwalled sheath had a conical tip which facilitated percutaneous entry and a preshaped distal bending which was particularly useful in the right ventricle for obtaining a steady and safe position in the direction of the interventricular septum. With respect to the histopathological information from an average of 4.2 biopsies per patient, a high clinical yield was gained in 28% of the patients, in the form of specific diagnoses and/or findings influencing subsequent therapy, and useful diagnostic information was gleaned from a further 53%. Thus, the procedure appears safe as a part of routine cardiac catheterization, and biopsy may contribute to a more accurate and clinically relevant classification of a substantial number of patients with suspected myocardial disease.