Study Design: Retrospective case series. Objective: In this study, we will identify if partial ACL tear is a common hidden cause of chronic pain and swelling of the knee in adults. And we will evaluate the effectiveness of arthroscopic selective bundle reconstruction in the management of this condition. Background: The Anterior Cruciate Ligament (ACL) has two anatomical bundles, the anteromedial (AM) and the posterolateral (PL), each bundle was named according to the site of its tibial insertion.[1] Partial ACL tear is evident and symptomatic when the percentage of the torn fibers is more than 50% of ligament fibers. AM bundle is more liable to injury than PL.[2] Symptomps of partial ACL tear may be giving way and instability but persistent knee pain and swelling is often present as the main presenting symptom of the patient due to micro-instability of the knee, The primary complication of longstanding partial ACL tears is early knee degeneration.[3] Patients and Methods: 37 patients with chronic knee pain and swelling, related to activity and resistant to non-operative treatment, underwent diagnostic knee arthroscopy. In 3 cases diagnostic arthroscopy revealed osteochondral ulcers of the medial compartment of the knee managed by drilling and 4 cases showed non-specific synovitis managed by arthroscopic synovectomy. 30 of these patients, who were included in this study, were found to suffer from partial ACL tear and underwent single bundle reconstruction, 18 of them underwent selective AM bundle reconstruction while preserving PL bundle. Twelve patients underwent selective PL bundle reconstruction with AM bundle preservation. Semitendinosus tendon graft was utilized for all reconstructions. The femoral side was always fixed with an adjustable loop (Zimmer), and the tibial side with a biodegradable interference screw. Lysholm score[4] was used to assess the outcomes. Patients with severe degenerative disorders, lower limb mal-alignment and multiple ligamentous injuries of the knee were excluded from our study. Results: Marked decrease in the knee pain and swelling postoperatively. The preoperative score had a mean value of 66.17±10.39. At 2 years, the postoperative score was 96.1±6.71, indicating a highly statistically significant improvement (P value 0.001). Hypothesis: Partial ACL tears are the commonest hidden cause of pain and swelling of the knee among young adults. Conclusion