The solitary pulmonary nodule is a commonly encountered radiologic finding that might represent lung cancer, particularly in individuals at high risk to develop lung cancer. Morphological characteristics including lesion size, contour and edge, calcification and nodule density, and contrast enhancement may be helpful in the attempt to differentiate malignant from benign nodules. In certain instances, the combination of morphological characteristics indicates a specific diagnosis, sometimes precluding the need for further workup. Growth rate is of paramount importance in evaluating the nature of a solitary pulmonary nodule, and 2-year stability is traditionally considered an indicator of benignity. Screening for lung cancer using low- dose CT has led to the detection of small pulmonary nodules in a large number of high-risk individuals. In the workup of these nodules, lung cancer, when present, should be diagnosed as early as possible, while unnecessary surgery and needle biopsies should be avoided. Computer analysis may prove to be a valuable tool in the evaluation of these nodules. Copyright (C) 2000 by W.B. Saunders Company.