A rare case of a large lateral meniscal cyst of the knee.

Lateral meniscal cysts are usually small asymptomatic lesions, and to find a large cyst with extensive bony changes arising from this cyst is exceedingly rare. These changes are usually erosive defects of the tibia. Dull pain with an associated mass are classical findings. A 47-year-old man developed a symptomatic lateral meniscal cyst that eroded a large part of the lateral tibial plateau. Arthroscopy and arthrotomy with cyst excision was required in this case and led to the amelioration of his symptoms.