This report has discussed the use of an angiographic severity index for pulmonary embolism in the analysis of pulmonary angiograms from 160 patients admitted to the Urokinase-Pulmonary Embolism Trial. Angiographic studies done before and after treatment with urokinase or heparin were analyzed independently by three radiologists who had no knowledge of treatment assignment. A subjective evaluation and a subsequent objective recording of abnormalities were made. The computation of a severity index was based on a numerical grading system for definite abnormalities specific for pulmonary embolism (intraluminal filling defects and vascular obstructions). The procedure for computing the severity index was simple enough that recorded abnormalities were coded and punched by a statistical clerk and analyzed by computer. Correlations between subjective evaluations and objective severity indices were high. There was excellent agreement among the three radiologists for both subjective and objective methods. Highly significant treatment differences were detected. The angiographic severity index might help to classify patients, predict prognosis, plan clinical studies, and assess treatment effects.