Objective: To evaluate the potential of spiral computed tomography (CT) densitometry of the lung to assess segmental perfusion defects in patients with acute pulmonary embolism. Materials and Methods: Ten patients with known segmental or lobar perfusion defects on ventilation/perfusion scintigraphy and with normal findings in the contralateral lung segment underwent spiral CT of the thorax before and after the administration of contrast material. Regions of interest were defined in 14 segments with normal perfusion and in 14 segments with reduced perfusion. Three consecutive densitometry measurements were performed in each segment. Results: Those segments with reduced perfusion showed a significantly lower mean CT value on the enhanced scans (−813.4±57.1 Hounsfield units (HU) vs −794.0±44.8 HU, P=0.01) and a significantly decreased contrast enhancement (12.3±18.2 HU vs 29.8±16.6 HU, P<0.01) when compared to segments with normal perfusion. Measurements from the unenhanced CT scans were not statistically different between segments with reduced and normal perfusion. Conclusions: Spiral CT densitometry allows the assessment of at least segmental perfusion defects in patients with acute pulmonary embolism.