Unicompartmental knee replacement is an attractive concept that offers several potential advantages over bicompartmental or tricompartmental replacement, including preservation of bone stock, of the anterior and posterior cruciate ligaments, of the patellofemoral joint, and of the normal opposite compartment. Of our first 100 consecutive knees that had unicompartmental arthroplasty for osteoarthritis and were followed for two to six years (average, three and one-half years), eighty-eight were medial and twelve were lateral replacements. The ages of the patients at the time of operation ranged from forty-six to eighty-five years, with an average age of seventy-one years. At follow-up, pain relief was good to excellent in 92 per cent of the knees. The average amount of flexion was 114 degrees (range, 90 to 140 degrees); one-half of the knees had at least 120 degrees of flexion. The average flexion contracture was 1 degree. There were no infections and no peroneal palsies. At the time of writing, three failures had required revision. Radiolucent lines at the bone-cement interface were present around 8 per cent of the femoral components and 27 per cent of the tibial components. Two femoral components subsided in obese patients. There was no tibial loosening in the series. The most common complications, per anserinus bursitis, occurred in 12 per cent of the knees and was treated satisfactorily by injection of local anesthetics and steroids. Surgical technique must be precise to prevent subluxation of the tibia on the femur due to either improper positioning of the components or too tight a fit (too much pressure) between them. We view unicompartmental replacement as an attractive alternative for the treatment of unicompartmental osteoarthritis in elderly patients. However, we are strong advocates of proximal tibial osteotomy for unicompartmental osteoarthritis associated with a varus deformity in selected patients.