This study developed and validated an easily administered method of predicting a patient's risk of needing extended inpatient rehabilitation after hip or knee arthroplasty. Seven factors generated by experts and from the literature were shown to be statistically significantly related to discharge destination (P</=.001). Factor weightings derived from a logistic regression equation and tested on the first 520 cases were used to devise a scoring method. This method was validated using a further 130 cases and the Risk Assessment and Predictor Tool (RAPT) was formulated. The RAPT identified 3 levels of risk of needing extended inpatient rehabilitation after hip or knee arthroplasty, with an accuracy rate of 89% for those most at risk.