OBJECTIVES: To quantify the frequency of and excess cost associated with foreign objects (e.g., surgical instruments and sponges) left in the body during total joint arthroplasty (TJA). METHODS: Data from the 2009-2012 National/Nationwide Inpatient Samples (NIS) were analyzed using descriptive (i.e., frequencies and means) and bivariate statistical methods (i.e., t-test, chi-square, and their non-parametric equivalents). Patients that underwent TJA were identified through International Classification of Diseases, 9th Revision (ICD-9) procedure codes 81.51 (hip TJA) and 81.54 (knee TJA). Patients with retained foreign objects were identified using ICD-9 external causes of injury code E871.0. NIS sample weights were used to generate national incidence estimates. Costs were estimated using the NIS' charge-to-cost ratio for each hospital. All costs are reported in 2014 dollars using Consumer Price Index adjustment factors. We were only able to quantify the frequency and excess cost associated with foreign objects identified and reported during the peri-operative period (i.e., initial hospitalization). RESULTS: Included in the NIS were 761,225 patients that underwent TJA during the study period. Of these patients, only 44 (0.01%) had a retained foreign object (20 hip TJA and 24 knee TJA); however, TJA patients accounted for 4.5% of all surgical patients with a hospital-reported retained object. These TJA patients underwent one additional procedure, on average, than those without a retained object (p<0.0001). We estimate the total annual frequency of these events at 55 TJA patients. The estimated average excess cost of these avoidable adverse events is $4,950 (p=0.0068). CONCLUSIONS: Despite considerable effort aimed at reducing the frequency in which foreign objects are inadvertently left in the body during surgery, these avoidable events continue to occur in TJA. While retained objects are rare events in TJA, these adverse events lead to excess cost and procedural burden for patients. Quality improvement efforts should continue unitl these events are eliminated.