The purpose of this prospective and randomized study was to compare rehabilitation with early range of motion (ROM) training vs immobilization following anterior cruciate ligament (ACL) reconstruction. Fifty patients, undergoing an ACL reconstruction with a bone-patellar tendon-bone graft, were postoperatively allocated randomly to either a plaster cast or a brace for 5 weeks. The brace group had ROM exercises from postoperative day 7. The commencement of ROM exercises was postponed 4 weeks for the plaster group compared to the brace group, but progressed subsequently with equal speed. There was no difference between the groups in the ROM of flexion or extension 20 weeks after the ACL reconstruction and later. Twenty-four months after surgery, the muscle strength deficit in the hamstring muscles (isokinetic measurements; percent difference, injured vs uninjured) was significantly larger in the brace group (mean +/- SD: 5.9 +/- 7.8%, P < 0.01) than in the plaster group (- 0.9 +/- 11.8%, NS) (brace vs plaster group, P < 0.05). Furthermore, there was also a tendency in the brace group to a larger strength deficit in the quadriceps muscle (brace: 11.1 +/- 13.2%, P < 0.001; plaster: 3.8 +/- 12.9%, NS) (brace vs plaster group, P= 0.07). There was no difference between the groups in the total sagittal knee laxity, as measured with an arthrometer, or in the subjective knee function or activity level (Lysholm score together with the Tegner activity level) between the groups. It is concluded that the postoperative treatment with early range of motion training after ACL reconstruction gave as good ROM, knee stability, subjective knee function and activity level as the treatment with immobilization. It is hypothesized that the larger strength deficit observed after rehabilitation with early range of motion training is secondary to the more intensive training and physical therapist involvement that was demanded in order to achieve full ROM following immobilization.