The hypothesis proposed in this study was that the initiation of active and passive knee motion within 48 hours of major intraarticular knee ligament surgery would not have the deleterious effects of increasing knee effusion, hemarthrosis, periarticular soft tissue edema, and swelling. We conducted a prospective study with randomized assignment of 18 patients into two groups: 9 patients in the "motion" group began 10 hours of daily continuous passive motion (CPM) on the 2nd postoperative day, while the remaining 9 in the "delayed motion" group used a soft hinged knee brace with knee hinges locked at 10 degrees of flexion and entered into the motion program on the 7th postoperative day. All knees were allowed full 0 degrees to 90 degrees of motion except for a total of seven knees with concomitant mensicus repairs and extraarticular reconstructions where 20 degrees to 90 degrees of motion was allowed, limiting the last 20 degrees of knee extension for the first 4 postoperative weeks to protect the repair. In all other respects, the rehabilitation program after surgery was the same for the two groups, including postoperative compression dressings, exercises, and weight-bearing status. Ten of the eighteen patients had acute ACL disruptions and 8 had chronic ACL insufficiencies. There was an even distribution of acute and chronic knee cases and of open and arthroscopic ligament procedures in the early and delayed motion groups. Associated surgery included four meniscus repairs, three medial collateral ligament repairs, and one lateral collateral ligament repair. Special suturing and fixation techniques were used at surgery to maintain the integrity of ligament and meniscus structures, allowing the surgeon to feel safe in subjecting the joint to early postoperative motion. The objective parameters measured were KT-1000 arthrometer measurements, Cybex isokinetic testing, girth measurements at four lower limb locations, range of motion goniometer measurements, postoperative pain medications, and days of hospitalization. Starting intermittent passive motion on the 2nd postoperative day did not increase joint effusion, hemarthrosis, or soft tissue swelling. In both motion groups, postoperative joint effusions were absent after the 14th postoperative day. There was no statistically significant difference in knee extension or flexion limits, pain medication used, or hospital stay in comparing the two knee motion programs. An important finding of this study was the significant decreases in thigh circumference that occurred within the first few weeks of surgery, which progressed despite a closely supervised inpatient and outpatient rehabilitation program.(ABSTRACT TRUNCATED AT 400 WORDS)