Thirty-seven patients with unicompartmental osteoarthritis were treated by replacement arthroplasty using the Marmor modular prosthesis and each patient was followed for at least two years. Despite rigid preoperative criteria for the selection of cases, the over-all results were definitely inferior to those seen after either bicompartmental or tricompartmental replacement arthroplasty. Complications included recurrent pain and degeneration of the other compartment, often associated with abrasion of the remaining cartilaginous surfaces caused by polyethylene particles. There was also more settling of the tibial component in these patients than in patients treated by bicompartmental replacement arthroplasty. Because of these findings, we no longer use unicompartmental replacement of the medial side of the knee. The results after the three lateral compartment replacements, on the other hand, were good, and use of this procedure for post-traumatic arthritis is still under investigation.