From 1986 to the present, 126 total knee revisions were performed by the authors. Twenty-five cases (20%) were revised for instability of the tibiofemoral articulation. Patellofemoral instability and those patients treated conservatively were excluded from this subset of patients. All patients were clinically evaluated using the Hospital for Special Surgery knee rating system and radiographically rated using The Knee Society scoring system. The average follow-up period was 28 months. Preoperative synovial fluid analysis showed a predominance of red blood cells (average, 64,000). Reasons for instability were ligamentous imbalance and incompetence, malalignment and late ligamentous incompetence, a deficient extensor mechanism, inadequate prosthetic design, and surgical error. All patients currently have stable knees with an overall improvement in clinical and radio-graphic scores. When evaluating a patient with a painful knee after total knee arthroplasty, this diagnosis should be considered. Careful physical examination, dynamic radiographs, and synovial fluid analysis should help to make a proper diagnosis. Treatment should aim to correct the cause of instability. The prosthesis chosen should compensate for the specific ligamentous deficiency present.