Facing an ever-increasing demand for diagnostic radiological examinations, the radiologist has turned to other means for assistance. In some cases this has been the utilization of trained technical assistants. New methods and techniques have been tested. In this learning process, simulated lesions can be useful in training of assistants. It can also be an integral part of the evaluation of their abilities to perform the required functions. Second, the study of radiographic images may be quantitatively explored as to their use in diagnosis. Previous radiographic simulations have made use of photographic and television simulations. However, greater quantitative control can be achieved by the utilization of the digital computer. This system provides a qualitative and quantitative control plus a high degree of flexibility throughout the system. Images and radiographs can be studied to determine the ability of the trained observer to perceive information as displayed in the traditional radiograph1. The purpose of this paper is to describe the system used to simulate lesions in the chest radiograms and the effect of shape on the detectability of these lesions.