The primary aim of this study was to investigate the potential benefit of stress radiographs for diagnosis of unstable total knee arthroplasty (TKA) and to identify clinically relevant cut-off values to differentiate between unstable and stable TKAs. Data of 40 patients with 49 cruciate retaining (CR) TKA who underwent stress radiographs as part of the diagnostic algorithm in a painful knee clinic were prospectively collected. Anterior and posterior stress radiographs were done in 90° and 15° flexion, varus-valgus stress radiographs in 0° and 30° knee flexion. Knee laxity was measured in mm and degrees by two independent observers using standardized landmarks. Intra- and inter-observer single measure intraclass correlations were between 0.92 to 1 and 0.89 to 1, respectively. For evaluation and investigation of the potential cut-off values, two groups of patients with and without revision surgery due to instability were compared. Radiographic measures of standardized z values according to the group without revision due to instability were used to calculate average and maximum laxity z-scores. Knees undergoing revision TKA due to instability showed significantly (p < 0.001) lower (KSS) pain/function scores (94±6.3, range 80-100; control group: 112±19.2, range 80-148) and total KSS scores when compared to the control group. The laxity values of patients with instability were significantly higher in terms of mean values (p < 0.01) when compared to the control group. The maximum laxity z-score showed the strongest difference between the groups (R2 = 0.26, p < 0.001). The following cut-off values indicating need of revision due to instability were established: in 90° (15°) flexion-anterior translation 5.2mm (22.4mm), posterior translation 16.6mm (13.2mm); varus stress in 0° (20°-30°) flexion-inlay gap 5.2mm (6.1mm) or joint angle 6.1° (6.8°); valgus stress in 0° (20°-30°) flexion-inlay gap 4.6mm (5.7mm) or joint angle 5.2° (7.1°). Standardized stress radiographs are helpful tools for diagnosis of instability after TKA. The established cut-off values help to guide decision making in this challenging group of patients. However, laxity values should not be considered as the only criteria for diagnosis of unstable TKA. IV.