Aims: Prospectivemulti-centre study evaluating minimally invasive unicondylar knee arthroplasty (UKA). Methods: Between 1997 and 2001 435 UKA were performed using the minimally invasive technique according to Repicci. 420 patients with an average age of 66 years were evaluated after a follow up of 32 weeks. In 96,8% the medial and in 3,2% the lateral compartement were involved. The clinical results were evaluated with the Knee Society Clinical Rating System (KSS) and correlated with Body-Mass-Index (BMI) and pain, rated on a Visual Analogue Scale (VAS). Radiographical scoring were evaluated according to Ahlback. Results: The results of the KSS show a signiþcant postoperative increase of average 73 points. 86,8% patients were conþdent with the treatment. The statistical analysis revealed correlation between pain and patients age. BMI shows signiþcant correlation with all other parameters. Conclusion: Minimally invasive unicondylar knee arthroplasty shows very good results, espeically for pain and patientsñ conþdence. BMI and arthrosis of the patella might have the gratest inßuence for the outcome. Deþcient pre- and postoperative extension might be an unsolved problem.