Objective: To determine if patients' level of effort (LOE) in therapy sessions during traumatic brain injury (TBI) rehabilitation modifies the effect of compliance with the 3-Hour Rule of the Centers for Medicare & Medicaid Services. Design: Propensity score methodology applied to the TBI-Practice-Based Evidence (TBI-PBE) database, consisting of multi-site, prospective, longitudinal observational data. Setting: Acute inpatient rehabilitation facilities (IRF). Participants: Patients (n=1820) who received their first IRF admission for TBI in the US and were enrolled for 3 and 9 month follow-up. Main Outcome Measures: Participation Assessment with Recombined Tools-Objective-17, FIM TM Motor and Cognitive scores, Satisfaction with Life Scale, and Patient Health Questionnaire-9. Results: When the full cohort was examined, no strong main effect of compliance with the 3-Hour Rule was identified and LOE did not modify the effect of compliance with the 3-Hour Rule. In contrast, LOE had a strong positive main effect on all outcomes, except depression. When the sample was stratified by level of disability, LOE modified the effect of compliance, particularly on the outcomes of participants with less severe disability. For these patients, providing 3 hours of therapy for 50%+ of therapy days in the context of low effort resulted in