Computer software was designed for classifying solitary pulmonary nodules (SPNs) into benign and malignant from their CT images, using image analysis methods. The system made use of three features, computed from the CT density matrix of the SPN, and a class-discriminating algorithm. System evaluation was performed on 51 histologically confirmed SPNs of indeterminate CT diagnosis. Overall classification accuracy in distinguishing benign and malignant SPNs was 90.2%, while 83.3% of the benign and 93.9% of the malignant SPNs were correctly classified. The proposed system may be of value to the radiologist in assessing the probability of malignancy in patients with a solitary pulmonary nodule.