BACKGROUND: Various guidelines for the management of low back pain have been developed to enhance the effectiveness and efficiency of care. Evidence that guideline-adherent care results in better health outcomes, however, is not conclusive. OBJECTIVE: The main objective of this study was to assess whether a higher percentage of adherence to the Dutch physical and manual therapy guidelines for low back pain is related to improved outcomes. The study further explored whether this relationship differs for the individual steps of the process of care and for distinct subgroups of patients. DESIGN: This was an observational prospective cohort study (2005-2006) in the Netherlands that included a sample of 61 private practice therapists and 145 patients. METHODS: Therapists recorded the process of care and the number of treatment sessions in Web-based patient files. Guideline adherence was assessed using quality indicators. Physical functioning was measured by the Dutch version of the Quebec Back Pain and Disability Scale, and average pain was measured with a visual analog scale. Relationships between the percentage of guideline adherence and outcomes of care were evaluated with regression analyses. RESULTS: Higher percentages of adherence were associated with fewer functional limitations (beta=-0.21, P=.023) and fewer treatment sessions (beta=-0.27, P=.005). LIMITATIONS: The relatively small self-selected sample might limit external validity, but it is not expected that the small sample greatly influenced the internal validity of the study. Larger samples are required to enable adequate subgroup analyses. CONCLUSIONS: The results indicate that higher percentages of guideline adherence are related to better improvement of physical functioning and to a lower utilization of care. A proper assessment of the relationship between the process of physical therapy care and outcomes may require a comprehensive set of process indicators to measure guideline adherence.