Limitation of motion following ACL reconstruction is a well-recognized and disturbing complication. The purposes of this study were to identify and characterize those patients who developed this complication from a series of 527 ACL reconstructions, determine etiologic factors, and make recommendations regarding prevention and management. The case group included 37 patients who required a manipulation under anesthesia because of failure to gain a satisfactory range of motion after an ACL reconstruction. Unsatisfactory motion was defined as a flexion deformity of 10 degrees or more and/or limitation of flexion to less than 120 degrees by 3 months following ACL reconstruction. The control group of patients were selected randomly from the overall series and all had a satisfactory range of motion. The cases and controls were then compared by analyzing these variables: age, sex, knee, time from injury to reconstruction, type of tissue used, meniscal abnormalities or surgery, repair of the medical collateral ligament, and postoperative immobilization and rehabilitation. The cases were followed up to assess the range of motion compared to the opposite knee at an average of 26 months postmanipulation. Thirty-seven patients (7%) underwent a manipulation under anesthesia, 9 of these (24.3%) also had an arthroscopic arthrolysis. Reconstructions done less than 2 weeks postinjury showed a statistically significant higher rate of knee stiffness. The same trend was also present for those reconstructed 2 to 6 weeks postinjury. All other variables failed to show a significant statistical difference. At followup, the average loss of extension was 4 degrees and loss of flexion 5 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)