Pigmented villonodular synovitis (PVS) leads hemoarthrosis as one of the representative signs. We report a rare case of PVS originating below the patella tendon, which limited the joint extension. A35-year-old man with first onset pair and locking of the knee, which was attributed to an extra-articular neoplasm based on findings of MRI. The tumor showed low intensity on T1-weighted images, and high/iso intensity on T2-weighted images. The tumor was successfully resected in 2002 by open surgical procedure and diagnosed as PVS. There have been no signs of recurrence for 15 months following the operation. MRI was very important for diagnosis. It should be considered that tumors, including PVS, may cause limitation of extension and locking of the knee.
