Initial clinical experience is presented, on the use of a prototype portal imaging system which is designed to provide diagnostic-quality images on-line at the accelerator. The system comprises a compact diagnostic X-ray unit mounted on the accelerator head, with its source in the isocentric plane exactly 37 degrees around from the therapy source. The image detector is an image intensifier with digital image storage/processing facilities. Images were taken at the accelerator of treatment fields in seven patients (pelvic, head and neck, chest fields) and these images were comparable in anatomical contrast to simulator radiographs. Two techniques for marking the therapy field onto the portal images were successfully demonstrated. One was to mark relevant corners or edges of the therapy light field on the skin using small Pb markers which become clearly visible in the 'diagnostic' portal image. The other was to record a separate 'therapy' image and through the software, extract and superimpose the field edges on to the 'diagnostic' image. The system and method proved fundamentally sound on criteria of image quality, geometric precision of rotation between therapy and imaging conditions, and its potential for development as a practical clinical tool.