Aims: Evaluation of unicompartmental knee arthroplasty (UKA) as an alternative to total knee replacement. Methods: 66 patients (76 knees) underwent UKA with the Allegretto prosthesis. The indication was age greater than 60 years, single compartment arthrosis, normal functioning anterior cruciate ligament, and varus/valgus deformity < 20û. Etiology was primary or secondary osteoarthrosis, the latter secondary to avascular necrosis. Mean age at operation was 70 years (range, 55 to 90 years). Patients were evaluated using the Hospital for Special Surgery (HSS) rating score. Eight patients (8 knees), with less than 12 month follow-up were excluded. 58 patients (68 knees) were available at þnal review with a mean follow-up of 65 months (range, 12 to 120 months). Results: The average HSS score was 87.7 (range, 55 to 99). There were 53 (78%) excellent, 12 (17.5%) good, 2 fair, and 1 poor result. 52 patients (76%) had a BMI > 25. There was no signiþcant difference in HSS score between these patients and those with BMI < 25. Six knees (8.8%) were revised, 4 in obese patients (BMI > 25), and 2 in the same patient (bilateral) with a BMI of 32.9 (100kg). All 6 patients had a satisfactory outcome after total knee arthroplasty. There were no infections. Conclusions: UKA for uni-compartmental knee osteoarthrosis in properly selected patients produces satisfactory results. Moderate weight obesity is not an exclusion criteria.