Periprosthetic joint infection is one of the most challenging complications of total joint arthroplasty. We evaluated the incidence of early-onset (less than 2 years) and late-onset (greater than 2 years) periprosthetic joint infection after primary total hip arthroplasty (THA). The Medicare 5% national sample data set (1997-2006) was used to longitudinally follow primary THA patients. Deep infections were identified with the International Classification of Diseases, Ninth Revision, Clinical Modification code 996.66. Kaplan-Meier survivorship curves were compiled with infection as the end point. Cox regression was used to evaluate patient and hospital characteristics. Eight hundred eighty-seven THA infections were identified from 39,929 THA patients. The incidence of infection was 1.63% within 2 years and 0.59% between 2 and 10 years. Comorbidities, sex, procedure duration, and socioeconomic status were found to be significant risk factors. This is the first study to establish the incidence and risk factors associated with early onset and delayed periprosthetic joint infection in the Medicare patient population.