Purpose: To examine relationships between self-report and performance-based measures of physical function in ICU patients randomized to standardized rehabilitation therapy (SRT) or usual care (UC). Methods: Physical function was assessed in 257 ICU patients using self-report (physical functioning scale of the SF-36 (SF-36 PFS)) and the functional performance inventory-short form (FPI-SF) as well as performance-based measures (Short Physical Performance Battery (SPPB)) and muscular strength (MS). Assessments were at hospital discharge, 2, 4, and 6 months. Results: Correlations between self-report and performance-based measures were not significantly different between the two groups. When examining the entire cohort, a significant, but weak, correlation (r = 0.286) was found between the SF-36 PFS and the SPPB. At 2 months, moderate correlations were found between self-report and performance-based measures. The SF-36 PFS and FPI were significantly correlated with the SPPB (r = 0.536 and 0.553, respectively) and muscular strength (r = 0.413 and 0.431, respectively). Similar associations were seen at 4 and 6 months in both groups. Conclusion: Self-report and performance-based measures of physical function appear to assess different constructs at hospital discharge. Following recovery, these measures converge, but indicate different constructs are being assessed. As such, both self-report and performance-based measures of physical function should be used with ICU patients.