Single-stage bilateral total knee arthroplasties (BTKAs) are frequently performed in arthritic patients. The primary goals of physical therapy following knee arthroplasty are to increase knee flexion (KF) and achieve unassisted transfers and ambulation. The purpose of this study was to determine the differences in KF and postoperative day (POD) at which unassisted functional milestones (UFMs) were achieved in BTKA versus unilateral total knee arthroplasty (UTKA) patients. Physical therapists measured KF with a standard goniometer for 149 consecutive UTKA and BTKA patients. Day of discharge and the UFMs of transfers, ambulation with walker, ambulation with cane, and stair climbing were monitored with a valid functional index. Functional milestones were defined as unassisted when a patient performed them without requiring the presence of another person. Independent t tests, corrected for multiple comparisons, were used to compare groups (P < .05). The 61 BTKA patients attained their first UFM, walker ambulation, later than the 88 UTKA patients (POD 5.5 +/- 1.9 and 8.7 +/- 2.8, respectively). This 3-day difference persisted across all remaining UFMs and discharge date. KF angles on the POD at which each UFM was achieved were significantly greater in BTKA patients only for the early milestones of transfers and walker ambulation. After the ninth POD, KF was identical in both groups. Therefore, KF cannot be the only factor that influence the rate of progression of UFMs to discharge, since despite possessing the same KF as UTKA patients did, BTKA patients achieved advanced UFM's 3 days later.