Ganglion cysts originating from the cruciate ligaments have been reported rarely. A 38-year-old woman developed symptoms of knee pain with 10 degrees loss of knee extension. Preoperative magnetic resonance imaging showed a well-demarcated cystic mass surrounding the posterior cruciate ligament so clearly that further examination was not recommended. Because examination under anesthesia confirmed full extension of the knee, we presumed that pain produced by compression caused the diminished extension, and that mechanical block was not the reason. During arthroscopic examination, a mass was impinged between the anterior cruciate ligament and the intercondylar notch when extension of the knee was attempted. The mass was resected and immediate improvement was noted. The patient had experienced the same episode in the contralateral knee and removal of a ganglion cyst on the cruciate ligament 10 years ago. At the latest follow-up she was completely symptom free in both knees without any sign of recurrence.