Computed tomography (CT) was used to examine 634 solitary pulmonary nodules (SPNs). Each lesion was assessed as benign or indeterminate on the basis of CT criteria. Benign nodules made up 44% of all SPNs and 58% of the 431 that were 2 cm or less in diameter. All malignant SPNs were assessed as indeterminate, and adenocarcinoma (42%) was the most common primary malignancy. A total of 176 (63% of benign SPNs) were correctly assessed as benign by CT. Ninety SPNs assessed as diffusely calcified were not so identified by conventional tomography at outside institutions. An SPN can be reliably assessed by CT as benign if it exhibits high attenuation values, exceeding a critical level and distributed diffusely throughout a CT section through the center of the lesion and a well-defined edge. Although 38 of 283 (13.4%) primary lung cancers contained localized calcification, there was no significant overlap with the diffuse calcification of benign lesions. Central carcinoid tumors may contain focal ossification, but such lesions may be recognized by noting the proximity of larger bronchi. Assessment of SPNs by CT is most effective for lesions 2.0 cm or less in diameter. For larger lesions, the frequency of benign disease was decreased (14.3% of 203), as was the percentage of benign SPNs correctly assessed as benign by CT (37.9%).