Portal images obtained of patients during irradiation with high energy photon beams have a poor sharpness and contrast due to the physical limitations of the equipment forming these images. Consequently, improvement of the images is desirable. An inverse filter method was investigated for this purpose and values of the parameters of the filter have been determined for various clinical irradiation conditions. Data on the line spread function (LSF) were obtained from the image of a 0.2 mm wide slit between tungsten blocks that were positioned at the isocentre in front of a polystyrene phantom. Slit images were made in a 60Co beam and in a number of 8 and 16 MV x-ray beams with a copper screen-film detector. These images were made for various isocentre to detector distances, field sizes and phantom thicknesses. All slit images and a number of clinical images were digitised both with a TV camera system and a densitometer which were connected to a computer. The main factors that affect the LSF are the radiation source size and the isocentre to film distance. The photon energy had only a minor influence. The thickness of the phantom and the field size, however, did influence the contrast in the image and therefore they determined, together with the digitising system, the noise to signal ratio parameter of the inverse filter to a great extent. Application of the inverse filter improved significantly the visual appearance of anatomical details in the portal images. In addition to image restoration, the contrast was enhanced by the application of a low frequency cut-off filter.