In a recent study of the corresponding author, it was found that PEEK bearing elements revealed high (irreversible) surface strains if they were loaded between steel sheets. Since this reflects the conditions in the practical application and because the rolling properties are dominated by the surface material, a more detailed analysis of highly strained PEEK was required. Hence, fatigue tests in the high stress tensile regime were conducted. The experiments were carried out on servo-hydraulic testing machines and during the tests the mechanical response of the specimens was recorded. Two material modifications of PEEK were investigated in the research: untreated PEEK (without heat treatment) and annealed PEEK which was modified using defined thermal conditions. The analysis of the recorded test data aimed on the distinction between cumulative material response (creep deformation, material hardening / softening) and spontaneous material response (material hardening / softening). At the highest stress levels, the cumulative response pretended material softening with increasing number of cycles. However, by examining the spontaneous material response which became stiffer with increasing number of cycles, it was shown that the cumulative softening was caused by time-dependent deformation processes.