Objective To examine the validity of the self-report Work-Disability Functional Assessment Battery (WD-FAB) physical function scales relative to clinician ratings of function and a performance-based functional capacity evaluation called the Physical Work Performance Evaluation (PWPE). Design Cross-sectional. Setting Outpatient rehabilitation. Participants Adults (N=50) participating in physical therapy for musculoskeletal conditions. Interventions Not applicable. Main Outcome Measures Patients completed the PWPE and the WD-FAB physical function scales including Changing and Maintaining Body Position, Whole Body Mobility, Upper Body Function, and Upper Extremity Fine Motor. The physical therapist also answered the WD-FAB questions on the patient’s physical functioning. The WD-FAB computer-adaptive test version administered up to 10 items for each scale. The PWPE produces ratings from 0 to 5 indicating overall Level of Work ability: 0 (unable to work); 1 (sedentary); 2 (light); 3 (medium); 4 (heavy); 5 (very heavy). The PWPE also produces Level of Work ability ratings in the Dynamic Strength, Position Tolerance, and Mobility subsections. Results Participating in the study were 50 patients with 1 or more conditions (shoulder, n=21; knee, n=16; low back, n=13; ankle/foot, n=10; neck, n=8; hip, n=7). The patient-based WD-FAB scores demonstrated moderate, statistically significant correlations with the provider proxy WD-FAB report (R=.49-.65). The WD-FAB Upper Body Function scale scores demonstrated moderate strength relationships with the PWPE overall ratings. The Whole Body Mobility and Changing and Maintaining Body Position scales did not demonstrate statistically significant relationships with the PWPE overall ratings. Conclusions We found moderate evidence for validity for the WD-FAB Upper Body Function, Whole Body Mobility, and Changing and Maintaining Body Position scales relative to clinician report and varied evidence relative to the PWPE in this clinical sample.