  We present three-dimensional models of accretion disks in U Gem - like
systems and calculate their Doppler tomograms. The tomograms are based on two
different assumptions concerning the origin of line emission from the disk. The
assumption of lines originating due to irradiation of the surface layer of the
disk by the central source leads to a better agreement with observations. We
argue that fully three-dimensional modelling is necessary to properly interpret
the observed tomograms.
