We quantitatively evaluated the clinical usefulness of CRT diagnosis by using previous and current chest images with simulated lung nodules. Sixty normal chest X-ray films consisting of previous and current chest images were digitized. Simulated nodules prepared by a computer workstation were added on lung fields of the 30 current images. Observer performance tests were carried out with six radiologists to evaluate the detection of simulated lung nodules in CRT and hard copy film with previous and current images, and with only current films. ROC analyses indicated that the detection accuracy in CRT diagnosis with current and previous images was greater than that in CRT diagnosis with only a current image. In addition, there was no statistically significant difference between CRT and hard copy film diagnoses if current and previous images were employed for the diagnosis of simulated lung nodules. We believe that CRT diagnosis is as useful as hard copy film diagnosis for the evaluation of pulmonary nodules by using previous and current chest images.