Numerous postoperative therapies have been advocated for the rehabilitation of patients who have undergone ACL reconstruction. The effectiveness of these various methods, many of which are based on sound scientific principles, has yet to be documented. The purpose of this study was to determine the efficacy of five commonly used rehabilitation programs. Five groups of 20 patients, all of whom underwent the same method of ACL reconstruction, were compared in order to determine the effects of the following treatments (some in combination): transcutaneous electrical nerve stimulation (TENS), immobilization in flexion, immobilization in extension, electrical muscle stimulation (EMS), and continuous passive motion (CPM). Clinical evaluation, volumetric thigh measurements, instrumented varus-valgus stress testing, KT-1000 arthometer (Medmetric, San Diego, CA) measurements, and Cybex II (Cybex, Division of Lumex, Ronkonkoma, NY) muscle evaluation were used to examine the patients. TENS did not reduce the amount of pain medication required, nor was there improvement in any other clinically measurable parameter of performance. There was no clear difference in stability between those treated in extension and those treated in flexion; however, since three patients who were treated in extension required manipulation, there may be some advantage to treating patients with early limited range of motion in flexion. EMS did not reduce atrophy but it did minimize strength decrease during immobilization. EMS also resulted in significantly greater range of motion than those treated with extension or flexion with early limited motion. Compared to all groups, EMS patients had a significant reduction in the incidence of patellofemoral crepitation.(ABSTRACT TRUNCATED AT 250 WORDS)