We report 2 cases of mucoid degeneration of the anterior cruciate ligament (ACL). Mucoid degeneration of the ACL is a very rare cause of knee pain. The first patient presented with extension block of the knee, and the second patient had simple knee pain. Both patients did not have obvious trauma history or clinical instability of the knee. After arthroscopic debridement of the lesion, both patients regained full range of motion of the knee and were pain-free at follow-up. Mucoid degeneration of the ACL is suspected when an apparently thickened and ill-defined ligament with increased signal intensity on all sequences in magnetic resonance imaging is identified in a patient with physically intact ligament and no obvious trauma history. Arthroscopic debridement of the mucoid degeneration of the ACL was safe and effective in these 2 patients.