The instantaneous and equilibrium isothermal die swell ratios of thermoplastic polyurethanes have been measured by two methods. The incidence of some structural, geometrical, and processing parameters has been analyzed. The experimentation reveals the atypical behavior of these polymers related to a structural evolution during processing that greatly depends on the residence time in temperature under shearing and that probably results from the cumulated effects of multiple recombinations and chain cuts. This thermo-mechanical degradation affects the rheological properties of these polymers. The maximal behavior variations are observed for short times and stabilize above a specific critical time. Taking into account the effect of these rheological alterations on the generating mechanisms of die swell allows us to interpret all the experimental results (atypical or not) in terms of magnitude and direction of variation of the phenomenon.