A method has been developed to determine the individual settings of the MLC leaves from the image of the beam portal. The portal image is analysed by a two-step approach: (i) The general orientation of the collimator leaves is determined. (ii) Next, a one-dimensional edge operator (generalized Laplacian) is applied in a scanline fashion to extract an array holding the image coordinates of the field edge. The prescribed position of each leaf is then compared with the position of the steepest gradient behind that leaf as determined from the portal image. To obtain adequate precision in the determination of the leaf positions, the dose rate dependency of the electronic image detector, the increased transmission through the leaf tips and flanges, and the properties of the edge detector used for image processing were investigated. From these studies, a set of correction terms were obtained that are used to correctly calculate the position of the field edge from the prescribed MLC leaf settings. If not corrected, a leaf position-dependent discrepancy in the range -0.5 to 1.0 mm will arise. The results from tests of this method, applying the correction terms, show that the leaf positions can be determined with a precision better than 0.1 mm (one standard deviation). The reproducibility of the leaf positions was found to be better than 0.1 mm.