In this study, the evolution of the thickness profile of UHMW-polyethylene films during sequential biaxial drawing is studied. The basic assumption is that a nonunifonn thickness distribution of the drawn film is the result of the amplification of initial imperfections. Finite element simulations of the drawing process have been performed in which the drawing of a piece of film with a geometric imperfection is considered. For these numerical simulations, a viscoelastic model is adopted. Although the model does not describe the behavior of UHMW-PE films in full detail, it is able to capture typical features of inhomogeneous deformation. The most striking observation is that severe localization occurs at the beginning of the second drawing step (i.e. in the direction transverse to the first step). In this stage of the drawing process, a localization band develops. These phenomena are also observed in validation experiments. A discrepancy between the simulations and the validation experiments is observed at the end of the drawing process. In the simulations, the deformation is stabilized again, and the resulting thickness profile is practically uniform. In the experiments however, the bad thickness profile at the beginning of the second drawing step is conserved up to the end of the drawing process. The simulations have been performed for different process parameters. Hence, it is possible to access the influence of these parameters on the evolution of the thickness profile. Furthermore, a suggestion is done to explain the deformation phenomena. The stability of the deformation can be determined from stress-strain curves during the biaxial drawing process. It is concluded that, although no absolute values are acquired, trends are predicted correctly and they can serve as quidelines for process control.