In the period from 1977 to 1981, eleven patients with a primary bone neoplasm and one with a bone abscess, located in the shaft or distal end of the femur or the proximal part of the tibia, were referred to the Massachusetts General Hospital Orthopaedic Oncology Unit. All had had diagnostic or therapeutic arthroscopy. For one of the patients no roentgenograms had been made prior to arthroscopy. For another, roentgenograms had been made but were not repeated prior to the arthroscopy three months later. In six patients the lesions were clearly evident on the roentgenograms; they were not reported in four patients, while in two patients the lesions were not considered to be a contraindication to arthroscopy. Two lesions were located in the femoral shaft and one was in the popliteal space, but they had not been noted by the surgeon or radiologist. In four patients the lesion arising from the bone was biopsied through the arthroscope, introducing tumor cells into the joint and theoretically causing synovial seeding of the lesion. The problems raised by this study are obvious to all orthopaedic surgeons who perform arthroscopy. Any patient who is thought to have an intra-articular lesion must first be fully evaluated by history, physical examination, and, most importantly, appropriate biplane roentgenograms prior to the performance of any procedure. If a lesion arising from the bone is encountered during an arthroscopic procedure, the lesion must be biopsied not transsynovially, but through a separate extracapsular approach.