Computed tomography (CT) with thin sections (2-5 mm) was used to assess tissue density in 91 apparently noncalcified pulmonary nodules in 88 patients. The study included 45 primary lung malignancies and 13 metastases proven by subsequent lung biopsy or thoracotomy. There were 33 benign lesions of which 13 were biopsy proven. The other 20 nodules were presumed benign on the basis of serial radiography showing no growth. A representative CT number based on the 32 most dense voxels was calculated for each lesion. For the primary malignancies, the mean representative CT number was 92 H, with a standard deviation of 18 H. The highest representative CT number for any malignant lesion was 147 H. Of the 33 benign lesions, 20 had a representative CT number of 164 H or greater. It is presumed that nodules with representative CT numbers of 164 H are benign, and that diffuse calcification likely accounts for the higher CT number of some benign lesions. CT has proven to be more objective and more sensitive than standard tomography in assessing the density of pulmonary nodules.