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When mosaicking different DEMs systematic vertical and horizontal errors can produce visible discontinuities along the borders of the overlapping areas. Standard mosaicking procedures try to reduce only the inconsistencies at the boundaries of the areas of overlap and provide results where discontinuities are no more clearly visible, but do not remove the systematic errors that caused the artefacts. The proposed technique exploits the information contained in the area of overlap between different DEMs in order to reduce horizontal and vertical systematic errors also outside these areas. After removal of the systematic errors, more standard mosaicking methods can be used in order to reduce random noise and fill areas where data are missing. The proposed fusion approach can be generalized for mosaicking other kinds of images in addition to DEMs. Homogeneous DEMs of high quality at national scale, using the proposed technique for the integration of DEMs obtained from SRTM SAR-X and ERS SAR tandem data, have been obtained and validated in the framework of the DUDES project funded by ESA. The obtained DEM has been validated by comparison with high resolution DEMs and largely fulfill DTED2 specifications. Moreover, the accuracy of the obtained DEM is better than those of the single SRTM-X and ERS DEMs used as input for the fusion.