A simple method was devised to determine the characteristic curve of image intensifier (II)-TV digital imaging systems, which relates the output pixel value to the input relative x-ray intensity. To provide a wide range of x-ray intensities incident on the II, we used an aluminum stepwedge consisting of nine steps with thickness increments of 6.3 mm, together with an 0.81-mm-thick copper plate. The x-ray field was narrowly collimated to the area occupied by the stepwedge in order to reduce the effect of veiling glare. The relative x-ray intensities transmitted through each step of the stepwedge were determined by using screen-film systems. The gradient curve of the system was derived from the slope of the characteristic curve. Results obtained with a Siemens Digitron 2 system showed that its characteristic and gradient curves depended upon the matrix size used, but did not change with the II field size. The validity of the characteristic curve was demonstrated by measurement of iodine attenuation curves obtained with the II-TV digital system at different exposure levels.