AIM: To assess changes over time and determine the minimally important difference (MID) in the Oral Impact on Daily Performances (OIDP) index for patients with severe generalized periodontitis receiving periodontal treatment. METHODS: This study was nested in a larger randomized controlled trial and consisted of 45 consecutive subjects of the larger trial (17 receiving intensive and 28 receiving conservative periodontal care). The OIDP index assessed impacts on quality of life (QoL) at baseline and 1 month after treatment. Repeated-measures ANOVA was used for comparison over time and between treatment groups. To estimate the MID, two subjective global transition scales, related to periodontal and oral health, respectively, were used as anchors, whereas the effect size (ES), standardized response mean and standard error of measurement were also calculated. RESULTS: The mean OIDP score after treatment was significantly lower than at baseline, indicating improvement in QoL, but there were no differences between treatment groups. Based on an agreement between different methods, the MID of the OIDP index was around five scale points and corresponded to a moderate ES. CONCLUSION: The MID for the OIDP index among patients treated for severe generalized periodontitis provides meaning to change scores and facilitates interpretation of findings. Copyright 2010 John Wiley & Sons A/S.