Background. Clinical practice guidelines (CPGs) are not readily implemented in clinical practice. One of the impeding factors is that physical therapists do not hold realistic perceptions of their adherence to CPGs. Peer assessment (PA) is an implementation strategy that aims at improving guideline adherence by enhancing reflective practice, awareness of professional performance, and attainment of personal goals. Objective. The purpose of this study was to compare the effectiveness of PA with the usual case discussion (CD) strategy on adherence to CPGs for physical therapist management of upper extremity complaints. Design. A single-masked, cluster-randomized controlled trial with pretest-posttest design was conducted. Intervention. Twenty communities of practice (n=149 physical therapists) were randomly assigned to groups receiving PA or CD, with both interventions consisting of 4 sessions over 6 months. Both PA and CD groups worked on identical clinical cases relevant to the guidelines. Peer assessment focused on individual performance observed and evaluated by peers; CD focused on discussion. Outcomes. Guideline adherence was measured with clinical vignettes, reflective practice was measured with the Self-Reflection and Insight Scale (SRIS), awareness of performance was measured via the correlation between perceived and assessed improvement, and attainment of personal goals was measured with written commitments to change. Results. The PA groups improved more on guideline adherence compared with the CD groups (effect=22.52; 95% confidence interval [95% CI]=2.38, 42.66; P=.03). The SRIS scores did not differ between PA and CD groups. Awareness of performance was greater for the PA groups (r=.36) than for the CD groups (r=.08) (effect=14.73; 95% CI=2.78, 26.68; P=.01). The PA strategy was more effective than the CD strategy in attaining personal goals (effect=0.50; 95% CI=0.04, 0.96; P=.03). Limitations. Limited validity of clinical vignettes as a proxy measure of clinical practice was a limitation of the study. Conclusions. Peer assessment was more effective than CD in improving adherence to CPGs. Personal feedback may have contributed to its effectiveness. Future research should address the role of the group coach.