Three-dimensional, non-isothermal, polymer melt flow in sudden expansion is numerically investigated. The main goal of the paper is to study the general features of the flow field. The generalized Newtonian formulation is adopted, and the mathematical model corresponds to the laminar, incompressible Navier-Stokes equations. A second order finite difference scheme is used to discretize the governing equations in a collocated mesh. The non-Newtonian flow behaviour is modelled by the Cross constitutive relation, in which temperature effects are accounted for. The simulations show that, despite the high viscosity exhibited by polymer melts, a complex three-dimensional flow structure is found close to the expansion section, characterized by a spiral motion. The results also indicate that viscous dissipation causes limited effect in temperature rise and viscosity variations. The latter was found mostly affected by the shear rate. (C) 2009 Elsevier Ltd. All rights reserved.