False aneurysms caused by osteochondromas piercing an artery are apparently rare, although we have seen two cases within a six-month period. Other complications of osteochondromas such as chondrosarcoma, fracture, nerve paralysis and bone deformity are well enough recognized to be included in standard texts. We feel that perforation of an artery by an osteochondroma may not be as rare as the literature would indicate and we agree with previous authors that they should be prophylactically removed when they are discovered along the posterior part of the knee, especially when they arise from the distal part of the femur. Preoperative or operative arteriography is of value when the diagnosis is unclear or distal circulation is not apparent.