Loss of motion and knee extension weakness are recognized as significant complications following anterior cruciate ligament (ACL) reconstruction. The purpose of this study was to determine 1) what degree of preoperative motion loss represents a risk for postoperative motion problems and 2) if preoperative weakness (deficit > or = 20%) affects return of strength following surgery. Measurements of range of motion and strength were made on 102 patients (56 men, 46 women; age = 31 +/- 1 years) within 2 weeks prior to ACL reconstruction (preop) and repeated 6 months following surgery (postop). Thirteen of 40 patients (33%) lacking > or = 5 degrees preop, eight of 20 patients (40%) lacking 1-4 degrees preop, and three of 42 (7%) patients with full extension preop had > or = 5 degrees loss 6 months postop (p < 0.001). Thirty-two of 39 (82%) patients with normal strength preop had weakness 6 months postop. Forty of 51 (78%) patients with preop knee extension weakness still had weakness 6 months postop. Preop strength was not a good predictor of residual weakness following ACL reconstruction. The magnitude of the preop extension loss appears not to be a risk factor. It is the presence or absence of full extension equal to the contralateral leg that identifies risk for postop problems regaining extension.