Many arthroplasty outcome studies use administrative data to evaluate complications and mortality. Most databases use International Classification of Diseases, Ninth Revision or Current Procedural Terminology codes, which lack laterality information. This study determined the frequency with which a second operation occurs on the same side after total hip replacement (THR) and total knee replacement (TKR) and identified variables associated with ipsilateral reoperation. In a hospital-based sample of patients undergoing multiple THR or TKR, the side of the index operation was compared with the side of the subsequent operation. Concordance was defined as the percentage of same-side reoperations. Overall concordance was 23% for THR and 22% for TKR, suggest- ing most reoperations were on the contralateral hip or knee. This study provides estimates of misclassification of reoperation after lower extremity arthroplasty when conducting orthopedic research with adminis- trative data. Studies using these data should be aware of this limitation, and efforts should be made to limit reoperation to revision THR and TKR.