In 49 patients in whom gated equilibrium ventriculography and cardiac catheterization were performed within a 6 day interval, total and fractional portions of global and regional right ventricular ejection fraction (RVEF) were correlated with pulmonary arterial systolic pressure. Pulmonary arterial systolic pressure was normal (30 mm Hg or less) in 27 patients (Group I) and elevated (31 mm Hg or greater) in 22 patients (Group II). The second-half regional RVEF was 38 +/- 8% (mean +/- standard deviation) with a range of 30 to 54% for Group I and 22 +/- 6% with a range of 13 to 32% for Group II. The difference between the means was statistically significant (p less than 0.001). Use of a second-half regional RVEF of 30% as the criterion of elevated pulmonary arterial systolic pressure resulted in a sensitivity of 0.86 and a specificity of 1.00. A power curve fit in which pulmonary arterial systolic pressure = 10.91 (second-half regional RVEF)-0.87 allowed accurate estimation (r = -0.85) of pulmonary arterial systolic pressure from the second-half regional RVEF. It is concluded that second-half regional RVEF may be used to accurately detect pulmonary arterial hypertension and to estimate its extent.