Revision total knee arthroplasty for failed unicompartmental replacement. The results in nineteen patients (twenty-one knees) who had a failed unicompartmental knee replacement followed by a revision total knee arthroplasty were evaluated. There were twelve excellent, four good, one fair, and two poor results. The interval between the unicompartmental replacement and the revision total knee arthroplasty ranged from eight months to eight years. At the time of the revision, a major osseous defect was found in sixteen knees (76 per cent). The duration of follow-up after the revision ranged from two to ten years. At the most recent follow-up examination, radiographs revealed at least one radiolucent line in thirteen knees (62 per cent). The technical difficulties associated with the revision operation are evidence that unicondylar arthroplasty is not a conservative procedure that allows a total knee arthroplasty to be done easily later. The results also do not support the argument that a revision performed after failure of a unicondylar arthroplasty is less technically demanding than one performed after a failed primary total knee arthroplasty. 