A 5 year review of factors instigating malpractice claims and likely to result in a payout. Possible lessons for the future. Background. During 2002-2007 over 300,000 patients underwent knee arthroplasty (KA) in England and Wales, from which 204 cases of litigation were processed costing in excess of £5million. The complications associated with primary KA are well documented, however those instigating litigation in the UK are not known. This study assessed trends in litigation over the past 5 years identifying instigating factors and success rates to highlight areas for further improvement in patient information and surgical management. Methods. Data from the NHS Litigation Authority on claims following KA unrelated to trauma between 2002 and 2007 were obtained and analysed. Results. 246 claims were made, 171 (70%) were settled of which 81 (47%) resulted in a successful claim. The total cost to the NHS was £5,257,228. The number of claims has increased from 26 in 2002 to 67 in 2007, while the rate of successful claims decreased from 58% to 9%. The three most common instigating factors were ongoing pain(24%), operator error(23%) and infection(10%). The factors with greatest successful claims were operator error(80%), vascular injuries(75%), failure of post-operative care(55%). Conclusion. Litigation claims following KA are increasing, although there rate of success is decreasing. Operator error, vascular injuries and failure of post-operative care are predictors of a successful claim. However failure to consent adequately, adhere to policies and standard practice can result in a successful claim. Protecting patients intra-operatively and maintaining high technical expertise while implementing policies and informed consent decreases the litigation burden to the NHS