This study sought to determine the clinical utility of a new catheterization criterion for the diagnosis of constrictive pericarditis (CP). The finding of early rapid filling and equalization of end-diastolic pressures obtained by cardiac catheterization are necessary for the diagnosis of CP, but these findings are also present in patients with restrictive myocardial disease (RMD). Enhanced ventricular interaction is unique to CP. High-fidelity intracardiac pressure waveforms from 100 consecutive patients undergoing hemodynamic catheterization for diagnosis of CP versus RMD were examined. Fifty-nine patients had surgically documented CP and comprised group 1; the remaining 41 patients with RMD comprised group 2. The ratio of the right ventricular to left ventricular systolic pressure-time area during inspiration versus expiration (systolic area index) was used as a measurement of enhanced ventricular interaction. There were statistically significant differences in the conventional catheterization criteria between CP and RMD, but the predictive accuracy of any of the criteria was <75%. The systolic area index had a sensitivity of 97% and a predictive accuracy of 100% for the identification of patients with surgically proven CP. The ratio of right ventricular to left ventricular systolic area during inspiration and expiration is a reliable catheterization criterion for differentiating CP from RMD, which incorporates the concept of enhanced ventricular interdependence.