The aim of this prospective study was to evaluate Spiral CT in the primary diagnosis of acute pulmonary emboli and for follow-up after thrombolytic treatment. Digital subtraction angiography of the lung was used as the reference method. 38 patients were subjected to both procedures. 79% of Spiral CT and 63% of DSA examinations were optimal. The two methods agreed in the diagnosis of thrombo-embolism in 30 patients and excluded it in eight patients. Spiral CT verified thrombi in a total of 213 cases; of these 23 were in a main pulmonary artery (11%), 88 in lobar arteries (41%), and 102 in segmental arteries (48%). DSA demonstrated 180 thrombi. 17% of the adherent and partially occlusive thrombi were not shown. 38 pulmonary infarcts were found in 18 patients. In 15 patients resolution of thrombi following thrombolytic treatment was shown by Spiral CT. Spiral CT is an excellent alternative to DSA and its use in the diagnosis of pulmonary emboli is entirely appropriate.