The flow of a polymer melt through a flat spiral distribution system used in "stacked" type dies was analyzed through three-dimensional finite element simulations, and predictions of the exit flow variation were compared to experimental measurements. Both isothermal and non-isothermal simulations were carried out, and the non-isothermal results were found to be in close agreement with the measured flow variation. It was found that for this particular flat spiral die, viscous dissipation is extremely crucial and may lead to generation of hot spots and to increased flow variation. Finally, the effect of material viscoelasticity on the flow field was simulated by using a CEF constitutive equation. It was shown that neglecting viscoelastic effects due to shear deformation, represented by primary and secondary normal stress coefficients, in the modeling of the flow in the flat spiral dies is a reasonable simplification. On the other hand, extensional behavior should be taken into account for a more suitable description of the leakage flows within this type of die.