Fifteen elderly patients (sixteen knees) were seen because of acute pain in the knee and tenderness to palpation over the medial aspect of the tibial plateau. Initially, plain roentgenograms showed a radiolucent area at the site of the tenderness in only nine of the sixteen knees. However, radionuclide bone scans showed focal increased uptake at the site of the tenderness in four of the seven remaining knees, and magnetic resonance images showed discrete areas of low signal intensity at the same site in the other three knees. Plain roentgenograms eventually showed the typical lesion in all knees. Progression of the symptoms led to a total knee arthroplasty in nine knees and to a unicompartmental replacement in three; a satisfactory result was obtained in all twelve knees. An operation was recommended for two other knees, but it was refused by the patients. The symptoms resolved spontaneously in the remaining two knees. A degenerative tear in the medial meniscus, which is a common finding in this age-group, was noted at the time of a later operation in the three knees that had not had a radiolucent area on the initial plain roentgenograms but that had had an area of low signal intensity on the magnetic resonance images. If osteonecrosis of the tibial plateau is not considered as a potential cause of pain in the knee, symptoms may be attributed to a tear in the meniscus and an unnecessary and unproductive arthroscopy may be performed.