This randomized double-blind comparative study was designed to investigate the safety and efficacy of a new nonionic monomeric contrast agent, Iopentol 350 mgI/ml, versus a nonionic monomeric contrast agent currently used, Iopamidol 370 mgI/ml, in cardioangiography. Blood pressure, heart rate, end-diastolic left ventricular and mean aortic pressure, ECG, adverse reactions and discomfort were the safety variables recorded; technical adequacy and diagnostic yield were the efficacy variables recorded. A hundred patients entered the trial and were subdivided into two groups of 50 patients each; all of them were included in the safety and efficacy assessments. Demographic data, general and background characteristics and procedural data were comparable in the two treatment groups. No significant difference in efficacy was observed between the two groups: diagnostic yield was optimal in 90% of the patients in the Iopentol group and in 88% of the patients in the Iopamidol group. Systolic blood pressure 30 min. after the examination showed, in both groups, a slight but statistically significant reduction relative to baseline values (-3.48 mmHg and -3.85 mmHg in the Iopentol and in the Iopamidol group, respectively), while no significant reduction was observed in diastolic blood pressure. A statistically but not clinically significant decrease in heart rate was observed in the Iopamidol group 30 min. after the examination (-5.05%), while this variable remained practically the same in the Iopentol group (-0.58%). No difference was found between the two groups relative to the incidence of discomfort following the injection. Other adverse reactions were experienced by 7 patients in the Iopentol group and by 9 patients in the Iopamidol group: they all promptly recovered after medical treatment, with no sequelae. All but one event in the Iopamidol group (chest and abdominal pain with ECG changes) were mild to moderate and mainly related to the procedure and to the underlying disease. In conclusion, both contrast agents are safe and effective for use in cardioangiography.