Ionic contrast agents currently used in cardiac angiography are well tolerated but have certain negative effects. In a five-center study of 273 patients undergoing cardiac catheterization, safety and efficacy of the new nonionic contrast agent, iohexol, were compared with that of the standard ionic material, sodium methylglucamine diatrizoate. Contrast injections were made into the left ventricle and left and right coronary arteries in all patients. Left ventricular, systemic arterial and pulmonary capillary wedge pressures, and electrocardiogram lead II were recorded for 2 minutes. Following the first right and first left coronary injection, systemic arterial pressure and electrocardiographic recordings were made for 1 minute. After each subsequent coronary injection, recordings were made for 20 seconds. Both agents were found to be safe and both provided good diagnostic information. Iohexol was seen to cause both less marked overall hemodynamic alterations and a lower incidence of significant changes in pressure and heart rate. Iohexol may have an important role in coronary angiography and left ventriculography, particularly in high-risk patients, because of its lesser effect on heart rate and blood pressure.