Context: There is lack of information related to quadriceps and hamstring strength recovery during the early period of rehabilitation after anterior cruciate ligament reconstruction (ACLR) using hamstring-tendon graft (HTG). Objective: To investigate quadriceps and hamstring isometric strength at 4-, 8-, and 12-wk time points after ACLR and to document the strength changes of these muscles over time. Design: Longitudinal study. Participants: 24 patients (age 28.1 +/- 8.1 y) who underwent unilateral single-bundle anatomic ACLR with 4-strand semitendinosus and gracilis tendon graft. Main Outcome Measures: The isometric strength of quadriceps and hamstring muscles was measured on an isokinetic dynamometer at a 60 degrees knee-flexion angle 4, 8, and 12 wk after surgery. Results: Quadriceps and hamstring strength significantly increased over time for both the involved limb (quadriceps F-2,F-46 = 58.3, P < .001; hamstring F-2,F-46 = 35.7, P < .001) and uninvolved limb (quadriceps F-2,F-46 = 17.9, P < .001; hamstring F-2,F-46 = 56.9, P = .001). Quadriceps and hamstring indexes significantly changed from 4 wk (QI 57.9, HI 54.4) to 8 wk (QI 78.8, HI 69.9) and from 8 wk to 12 wk (QI 82, HI 75.7) (P < .001); however, there was no difference between indexes at the 12-wk time point (P = .17). Conclusions: The results of this study serve as a reference for clinicians while directing a rehabilitation protocol for HTG ACLR patients to better appreciate expected strength changes of the muscles in the early phase of recovery.