To assess whether it is possible to distinguish benign from malignant solitary pulmonary nodules with digital techniques, a retrospective study of 68 patients with proved solitary nodules was performed. The conventional chest radiograph for each patient was digitized to 2,048 X 2,048 X 12 bits, and changes in the optical density within the nodule were analyzed. A number (the corrected gradient number) was then generated that reflected this variation. Striking differences were noted between 26 malignant nodules and 21 calcified granulomas. The technique was then applied to 21 benign nodules that had initially required thoracotomy or further study for diagnosis. In nine of these 21 patients (43%), the corrected gradient number allowed correct classification as a benign lesion.