Background: Pain intensity (PI) is a common outcome parameter in effectiveness studies on interdisciplinary multimodal pain therapy (IMPT), despite the fact that IMPT highlights dealing with rather than reducing chronic pain. Moreover, the measurement of pain intensity as a highly subjective experience is problematic. Patient participation is absolutely essential to examine the relevance of PI as a criterion of treatment success as well as to select/develop suitable measurement methods. Method: A qualitative multicenter study was conducted using focus groups with 69 patients (18–77 years; 80% female) at four different IMPT centers in Germany to discuss pain intensity as a therapy outcome parameter in IMPT, as well as the interpretability and feasibility of common measurement methods. Results: The discussions emphasized that PI is a relevant, but not the primary, outcome in IMPT for patients. Patients’ statements also demonstrate that there are some problems in measuring PI, for instance with regard to pain attacks. Conclusions: The focus group discussions suggested that, due to the highly subjective nature of PI, as well as (verbal) inaccuracies and a lack of standardization in common instruments, the measurement of pain intensity is a challenging task. These limitations should be taken into account in future studies.