The extrudate (die) swell of a high-density polyethylene (HDPE) melt was studied both experimentally and numerically under slip conditions. The excess pressure drop due to entry (entrance pressure drop), the effect of pressure and temperature on viscosity, and the slip effects on the capillary data analysis have been examined. Using a series of capillary dies having different diameters, D, and length-to-diameter L/D ratios, a full rheological characterization has been carried out and the experimental data have been fitted both with a viscous model (CarreauYasuda) and a viscoelastic one (the Kaye-Bernstein, Kearsley, Zapas/Papanastasiou, Scriven, Macosko or K-BKZ/PSM model). Particular emphasis has been placed on the effects of wall slip (significant for HDPE). It was found that viscous modeling underestimates the pressures drops (especially at the higher apparent shear rates and L/D ratios) and predicts virtually no extrudate swell. On the other hand, the viscoelastic simulations were capable of reproducing the experimental data well, and this was particularly true for the pressure drop. The prediction of viscoelastic extrudate swell presented a problem, since the simulations grossly overpredict it due to the highly elastic nature of the melt. This occurs despite the presence of severe slip at the wall, which brings the swell down considerably. At this point it is not clear whether this is due to the viscoelastic model used or other phenomena, such as sagging and/or cooling, when simply extruding in the atmosphere. (c) 2012 Wiley Periodicals, Inc. Adv Polym Techn 32: E369E385, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21285