BACKGROUND: There is no agreement for the performance assessment of patients who practice exercises.. (2 points to withdraw) This assessment is currently left to the physiotherapist's personal judgement. We studied the agreement among physiotherapists in rating patient performance during exercises recommended for chronic low-back pain (LBP). METHODS: A vignette-based method was used. We first identified ten exercises recommended for LBP in the literature. Then, 42 patients with chronic LBP participating in a rehabilitation program were videotaped during their performance of one of the ten exercises. A vignette was an exercise video preceded by clinical information. Ten physiotherapists from primary (4) and tertiary care (6) viewed the 42 vignettes twice, one month apart, and rated patient performance from zero (worse performance) to ten (excellent performance) by considering the position and duration of the contraction or stretching. Intra-class correlation coefficients (ICCs) and 95% confidence intervals (95% CIs) were computed to assess inter- and intra-rater reliability. RESULTS: The overall inter-rater agreement was fair (ICC 0.48 [95% CI 0.33-0.56]) but was better for stretching exercises (0.55 [0.35-0.64]) than strengthening exercises (0.42 [0.20-0.52]) and for tertiary-care physiotherapists (0.66 [0.54-0.76]) than primary-care physiotherapists (0.28 [0.09-0.37]). The intra-rater agreement was overall good (0.72 [0.57-0.81] to 0.88 [0.79-0.94]). It was better for stretching exercises (from 0.68 [0.46-0.81] to 0.96 [0.91-0.98]) than strengthening exercises (from 0.68 [0.38-0.84]) to 0.82 [0.56-0.92]). CONCLUSION: The agreement in rating patient performance of exercises for LBP is good among physiotherapists trained in managing LBP but is low among non-trained physiotherapists.