The Paprosky classification provides a straightforward algorithm for defining bone loss and directing treatment for femoral revision. The purpose of this study was to test the inter-observer and intra-observer reliability of this system. Four arthroplasty surgeons reviewed radiographs of 205 consecutive femoral revisions. For each radiograph, the pattern of femoral bone loss was classified by Paprosky type on two separate occasions. A kappa value was used to calculate the reliability, which demonstrated an inter-observer reliability of 0.61, indicating substantial agreement between surgeons. The intra-observer reliability for each of the 4 participating surgeons was 0.81, 0.78, 0.76, and 0.75, indicating substantial to almost perfect agreement. There is substantial agreement among experienced arthroplasty surgeons when using the Paprosky Classification to characterize femoral bone loss.