From 1983 to 1989, 147 unicompartmental knee arthroplasties (UKA) were performed. Eighteen knees (13%) have required revision: there were 11 (37%) Porous Coated Anatomic (PCA), three (8%) Microloc, and four (17%) Fibermesh UKAs. Retrieved implants were examined to determine the pattern and extent of polyethylene wear. Wear was severe in each series and was characterized by delamination, pitting, peripheral cracking, deformation, and abrasion. Polyethylene failure appeared to result from large localized stresses resulting from lack of conformity of the articular geometries of the prostheses. Progressive subluxation of the implants occurred that produced very high localized contact stresses at the periphery of the tibial component where the polyethylene was thinnest. The progression of osteoarthritis in these knees, particularly the attenuation of the anterior cruciate ligament, increased the tendency of these incongruous implants to subluxate. Although changes in the design and manufacturing of unicompartmental knee arthroplasties may increase the resistance of these implants to polyethylene wear, the progression of osteoarthritis at an unpredictable rate is likely to be associated with persistent polyethylene wear in this type of arthroplasty.