A hospital-based computer system was used to compare the inpatient costs of performing bilateral simultaneous sequential, staged, and unilateral total hip and knee arthroplasties. Bilateral simultaneous sequential total knee arthroplasty was 36% less costly than 2 unilateral total knee arthroplasties. Bilateral simultaneous sequential total hip arthroplasty saved 25% over the costs of performing 2 unilateral hip arthroplasties. Prosthetic costs range between 28% and 43% of the total costs of hospitalization. There was a significant correlation between hospital length of stay, morbidity, and total costs, but no correlation with patient age and sex except in the unilateral knee patients. Bilateral simultaneous sequential joint arthroplasty can save more than $10,000 for each total knee patient and more than $8,000 for each total hip patient.