The objective was to evaluate the helical CT (HCT) criteria that could indicate severe pulmonary embolism (PE). In a retrospective study, 81 patients (mean age 62 years) with clinical suspicion of PE explored by HCT were studied. The patients were separated into three different groups according to clinical severity and treatment decisions: group SPE included patients with severe PE based on clinical data who were treated by fibrinolysis or embolectomy ( n=20); group NSPE included patients with non-severe PE who received heparin ( n=30); and group WPE included patients without PE ( n=31). For each patient we calculated a vascular obstruction index based on the site of obstruction and the degree of occlusion in the pulmonary artery. We noted the HCT signs, i.e., cardiac and pulmonary artery dimensions, that could indicate acute cor pulmonale. According to multivariate analysis, factors significantly correlated with the severity of PE were: the vascular obstruction index (group SPE: 54%; group NSPE: 24%; p<0.001); the maximum minor axis of the left ventricle (group SPE: 30.2 mm; group NSPE: 40.4 mm; p<0.001); the diameter of the central pulmonary artery (group SPE: 32.4 mm; group NSPE: 28.3 mm; p<0.001); the maximum minor axis of the right ventricle (group SPE: 47.5 mm; group NSPE: 42.7 mm; p=0.029); the right ventricle/left ventricle minor axis ratio (group SPE: 1.63; group NSPE: 1.09; p<0.0001). Our data suggest that hemodynamic severity of PE can be assessed on HCT scans by measuring four main criteria: the vascular obstruction index; the minimum diameter of the left ventricle; the RV:LV ratio; and the diameter of the central pulmonary artery.