Background Evidence-based practice has become a major issue in physical therapy. Many evidence-based guidelines, however, are not used extensively after dissemination, and interventions aimed at increasing guideline adherence often have limited effects. Objective As a prerequisite for changing this situation, the aims of this study were to gain an in-depth understanding of the determinants of guideline adherence among physical therapists in the Netherlands and to evaluate the opportunities of a theoretical framework in this respect. Design and METHODS:/b> This observational study consisted of 3 focus group interviews (n=12, 10, and 8) between November 2002 and January 2003. Physical therapists were asked to discuss their opinions about and experiences with the Dutch guidelines for low back pain. Data were analyzed qualitatively using a directed approach to content analysis. Both the interview route and the analysis of the interviews were informed by Rogers' Diffusion of Innovations Theory. RESULTS: /b> Our study yielded in-depth insights into the various determinants of guideline adherence. Overall, the participants had rather unfavorable opinions about issues related to the dissemination of the guidelines (first phase of the diffusion process) and provided relatively little information on the subsequent adoption process (second phase of the diffusion process). The theoretical framework appeared to be a useful tool to properly structure the focus group interviews, to systematically analyze the data collected, and to determine that supplementary interviews would be necessary to cover the entire diffusion process. CONCLUSIONS:/b> Our findings indicated that the diffusion process of guidelines among physical therapists was not yet completed. The use of theory can provide added value to guideline implementation studies.