For prevention of limitation of range of motion due to arthrofibrosis of the knee joint after anterior cruciate ligament reconstruction, it is accepted in general that the operation should be delayed for about three weeks after injury. We studied the duration form operation to time of full range of motion, and analized the results in forty-eight patients who had undergone arthroscopic anterior cruciate ligament recostruction using the autologous bone-patella tendon-bone graft and early CPM, range of motion exercise during Feb. 1991 and June 1994. The results were as follows: 1. The average interval from injury to operation were 1.3 weeks in acute injury group, 6 weeks in subacute injury group and 20.7 months in chronic group. The average interval from operation to full range of motion were 7.1 weeks in acute injury group, 5.7 weeks in subacute injury group and 3.0 weeks in chronic group. 2. In acute injury group, 14(93.3%) of 15 cases showed full extesion and only 1 case(6.7%) showed the limitation of extension less than 5 degrees. In subacute injury group, 6(85.7%) of 7 cases showed full extension but1 case(14.3%) showed the limitation of extension less than 5 degrees. On the other hand in the chronic injury group, all the 26 cases(100%) of the patients showed full extension. 3. In acute injury group, 14(93.3%) of 15 cases showed full flexion and only 1 cases(6.7%) showed the limitatin of flexion less than 5 degrees. In subacute injury group, 6(85.7%) of 7 cases showed full flexion but 1 case(14.3%) showed the limitation of flexion less than 5 degrees. On the other hand in the chronic injury group, 23(88.5%) of 26 cases could flex to full range of flexion, and 3 cases(11.5%) revealed limitation of flexion less than 5 degrees. 4. The limitation of ROM mainly due to arthrofibrosis can be prevented by early range of motion execise and active rehabilitation in the acute or chronic anterior cruciate ligament reconstruction using the autograft bone-patella-bone graft. Although in acute patients the interval from operation to full ROM can be prolonged, the full ROM can be achieved after all.