The system for predicting tumour location from skin motion induced by respiration was designed to reduce the effects of target movement. Fluoroscopic studies on 34 sites in the lungs and 14 sites in the diaphragm were performed so that the motions of skin markers and organs could be observed simultaneously. While patients were lying down in the simulator with radio-opaque markers on their skin, fluoroscopic images both in the anterior-posterior (AP) view and in the lateral view were sent to an analysing computer and recorded. The results that showed a strong correlation (0.77+/-0.12) between the patients' skin and tumour movement, especially for the sites located in the lower lung fields or in the diaphragm. With the prediction from skin motion, the uncertainties of the position of tumours due to respiratory movement could be reduced by up to 1.47 cm in the lower lung fields in the superior-inferior (SI) direction. This study revealed that it is possible to trace the exact location of tumours in the lungs by observing skin motion in most cases (up to 88%).