This paper is concerned with pressure driven flow behaviour of molten viscoelastic polymers and reports experimental data and matching numerical simulation for a linear low density polyethylene (LLDPE) melt flowing within an entry and exit slit geometry. Pressure driven processing experiments were carried out using a multipass rheometer (MPR) and time dependent flow birefringence and pressure data were obtained for a series of different flow conditions. A matching numerical simulation was developed using a Rolie Poly constitutive equation with the additional factor of compressibility included. Most but not all theological parameters were obtained from rheometric viscoelastic measurements on the polymer. Using the non-stretching version of the constitutive equation, the simulation was able to capture many of the observed experimental features and demonstrated that the Rolie Poly equation was effective in describing the flow of the LLDPE tested. The sensitivity of both viscoelasticity and compressibility was demonstrated for the time dependent build up and relaxation of stress and in addition, the simulation was successfully able to predict an initially unexpected time dependent variation of the absolute pressure measured in the upper and lower barrel chambers of the MPR. (c) 2006 Elsevier B.V. All rights reserved.