OBJECTIVE: To assess physiotherapists' adherence to the Dutch guidelines for nonspecific low back pain, the motivational determinants related to guideline adherence, and the role of physiotherapists' awareness of their performance in this respect. STUDY DESIGN & SETTING: This was a cross-sectional survey among a random sample of 1,500 private practice physiotherapists in the Netherlands. The actual guideline adherence was measured by means of validated clinical vignettes and self-reported adherence by asking the physiotherapists to report their own level of adherence. The assessment of motivational determinants was based on a theoretical framework. RESULTS: The response rate was 31.5% (N=472). The average guideline adherence rate was 50.4% (SD=16.8). Only 38.5% of the physiotherapists had realistic perceptions of their personal performance. Awareness levels seriously interfered with the relationship between motivational determinants and actual guideline adherence. Actual adherence was mainly related to the perceived relative advantages and awareness of adherence to the perceived social norm. CONCLUSION: The moderating role of awareness in this study confirms the view that motivational determinants of a particular behavior can only be accurately assessed if people hold realistic perceptions of that behavior. Our approach illustrates the added value of a theory-based approach in guideline implementation studies.