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Complex dose-delivery techniques, as currently applied in intensity-modulated radiation therapy (IMRT), require a highly efficient treatment-verification process. The present paper deals with the problem of the scatter correction for therapy verification by use of portal images obtained by an electronic portal imaging device (EPID) based on amorphous silicon. It also presents an iterative method for the scatter correction of portal images based on Monte Carlo-generated scatter kernels. First applications of this iterative scatter-correction method for the verification of intensity-modulated treatments are discussed on the basis of MVCT- and dose reconstruction. Several experiments with homogeneous and anthropomorphic phantoms were performed in order to validate the scatter correction method and to investigate the precision and relevance in view of its clinical applicability. It is shown that the devised concept of scatter correction significantly improves the results of MVCT- and dose reconstruction models, which is in turn essential for an exact online IMRT verification.