Extrusion of a hot polymer melt through a cooler die zone substantially increases the extrudate swell of some thermoplastics. This effect was examined for commercial samples of low-density polyethylene, polypropylene, and polystyrene. Two conflicting effects come into play during extrusion of a thermoplastic. Colder melt temperatures promote increased extrudate swell, but the same conditions also facilitate molecular disentanglement and reduced melt elasticity and die swell. Since the extrusion process itself may affect the relation between die swell and melt temperature, laboratory-scale measurements for the design of processes like blow molding are better carried out with small-scale screw extruders than with capillary rheometers. For some applications it may be advantageous to use a polymer whose die swell is particularly responsive or unresponsive to die temperature variations. The procedure described in this article can be used effectively to monitor this characteristic.