OBJECTIVE: The aims of this study were to identify and quantify functional deficits between the involved and the uninvolved limb as early as 1 mo after ACL reconstruction by means of a sit-to-stand movement on two force platforms. DESIGN: Ground reaction forces were measured in 73 patients who underwent ACL reconstruction, 47 with patellar tendon autograft and 26 with hamstring tendon autograft (ACL-H), and in 22 controls, who stood up from a seat as fast as possible. Side-to-side limb symmetry index (LSI) was calculated for peak force and rate of force development. RESULTS: The LSI of vertical peak force was 59% in the patellar tendon autograft group and 68% in the ACL-H group, which were both lower than the 95% LSI of the controls (P < 0.001). The LSI of rate of force development was 27% in the patellar tendon autograft group and 48% in the ACL-H group, which were both lower than the 97% LSI of the controls (P < 0.001). After 2 mos, only in the ACL-H group, the LSI of both vertical peak force and rate of force development reached 88% and 80%, respectively, thus approaching the 96% and 92% LSI of the controls. CONCLUSIONS: Sit-to-stand is effective for measuring functional deficits as early as 1 mo after ACL reconstruction. After 2 mos from surgery, the ACL-H group but not the patellar tendon autograft group approached the LSI of the control group, which may be attributed to differences in the graft.