In the United States, about 8 million people have a history of cancer, either cured or in the process of being treated, and predictions indicated that 1.2 million new diagnoses of invasive cancer would be made in 1999 (1). Extrathoracic and thoracic malignancies frequently metastasize to the lung parenchyma as pulmonary nodules, and the most sensitive diagnostic imaging modality for detecting such nodules is computed tomography (CT) of the thorax. To assess disease progresion or regression with therapy when metastatic disease to the lung is discovered, precise quantitative and reproducible analysis is required of what is often a large number of nodules. Computer schemes are therefore needed that help (a) improve nodule detection by the radiologist, (b) quantify nodule volume and number, and (c) assess change in nodule number and diameter.