OBJECTIVES: Due to response shift (RS) the QoL scores over time may be biased. RS was investigated in morbid obese patients undergoing two types of stomach restriction surgery. METHODS: Study design was a randomised controlled trial (RCT) of VBG (open surgical procedure) versus LapBand-surgery (laparascopic surgical procedure) (N = 60), with a pre-test before surgery and post-tests 3 (T1) and 6 (T2) months after surgery. QoL was assessed using the EQ-5D Visual Analogue Scale (EQ-VAS).) RS was examined by a then-test: a retrospective pre-test obtained at the same time as the post-tests. RS was computed by subtracting the pre-test from the then-test score. Then-test change was computed by subtracting the then-test scores from post-test scores. Conventional change was computed by subtracting the pre-test score from the post-test score. Repeated-measures ANOVA was used to examine differences. RESULTS: Thirty-six patients (27 female, mean age 39 ± 8.7) underwent VBG and 24 patients (22 female, mean age 36 ± 7.8) underwent LapBand. EQ-VAS increased after surgery in both the LapBand-group (pre-test: 65 ± 16.9; T1: 78 ± 15.7; T2: 76 ± 18.8) and the VBG-group (pre-test: 60 ± 21.7; T1: 78 ± 16.8; T2: 80 ± 17.4) (p = 0.000, p = 0.000), but the conventional change was not different between the groups (p=.144). RS was larger in the VBG-group (T1: -17.5 ± 16.9; T2: -18.3 ± 22.8) than in the LapBand-group (T1: -9.38 ± 22.6; T2: -8.65 ± 21.0), but the between-group difference was not statistically significant (p = 0.798). Then-test change was statistically significantly different between the groups (p= 0.034). CONCLUSION: As a result of the between-group difference in RS the then-test change was, in contrast to the conventional change, statistically significantly different between the groups. Since RS was larger in the group with a larger treatment effect (VBG-group), RS may be not only be related to the direction, but also to the size of the treatment effect. This may have implications for the interpretation of QoL changes in RCTs.