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Objectives There is evidence that treatment expectations predict treatment outcomes in pain management and other clinical conditions. However, translating these insights into clinical practice remains challenging: it is difficult to measure the multifaceted construct of expectations across diverse medical and psychological treatment modalities. Furthermore, little is known about how prior treatment experiences shape different expectation domains. A unified assessment approach is lacking, limiting comparability across studies and clinical contexts. The Generic Rating Scale for Previous Treatment Experiences, Treatment Expectations, and Treatment Effects (GEEE) seeks to overcome these limitations. The present study aims to explore the GEEE in a naturalistic clinical sample of people seeking treatment for chronic pain, which may provide preliminary evidence for its validity and applicability. An additional exploratory aim is to examine whether the GEEE is suitable for predicting treatment outcomes longitudinally in this clinical setting.Design Prospective longitudinal observational study with three measurement time points (baseline, 3 weeks and 16 weeks).Setting Specialised outpatient pain treatment centre in Germany.Participants The baseline sample comprised 219 patients with chronic pain, follow-up data were available from 140 participants at 3 weeks and 108 participants at 16 weeks, constituting the longitudinal subsamples.Primary and secondary outcome measures Primary outcomes were prior treatment experiences, current treatment expectations and treatment effects, which were assessed using the GEEE, as well as clinical outcomes of pain intensity and pain-related disability. Secondary measures included desire for pain relief, depression and anxiety symptoms, which were analysed in correlational tests to assess construct validity. Trajectories of treatment expectations and clinical outcomes were examined longitudinally, and it was assessed if baseline expectations predicted clinical outcomes over time.Results Regarding validation, at baseline, improvement expectations correlated weakly with expectations of worsening and side effects (Rho≈−0.14 to −0.15, p=0.028–0.034). Negative previous treatment experiences were associated with current expectations of worsening and side effects (Rho≈0.35–0.41, p≤0.002). The GEEE items on current treatment effects correlated with changes in clinical outcomes (pain intensity and pain-related disability; |r|≈0.17–0.56, all p≤0.043). Regarding prediction, treatment expectations remained stable, while pain intensity (η²G=0.076, p<0.001) and pain-related disability (η²G=0.037, p<0.001) decreased over time. Regression models predicting subjective improvement were significant at 3 weeks (R²=0.18) and 16 weeks (R²=0.29), with baseline improvement expectations emerging as a significant predictor. Models predicting pain intensity and disability at 16 weeks were also significant (R²≈0.50–0.59), and higher baseline improvement expectations were independently associated with better outcomes.Conclusion We found supporting evidence for the validity of the GEEE and its applicability in longitudinal clinical research. Improvement expectations as measured with the GEEE at baseline predicted better treatment outcomes, while previous negative treatment experiences correlated with current treatment expectations. The findings underscore the value of assessing and addressing expectations and prior treatment experiences to optimise treatment outcomes.