The study aims to delineate the deep infection rates and infection risk factors for primary total knee and total hip arthroplasty patients. A retrospective review was conducted on 6108 patients from 1991 to 2004. The deep infection cases were compared to the noninfected cohort whereby infection risk factors were identified. Of the 8494 joint arthroplasties, 43 (0.51%) developed a deep infection (30 total knee arthroplasties, 13 total hip arthroplasties). Patients with a body mass index greater than 50 had an increased odds ratio of infection of 21.3 (P < .0001). Diabetic patients were 3 times as likely to become infected compared to nondiabetic patients (P = .0027). Simultaneous bilateral total joint arthroplasties were found to have developed infection 3 times less frequently than those performed as unilateral procedures (P = .0024). The average age in our infection cohort was 64.3 and 68.4 in the noninfected cohort. In this retrospective review study, obesity, diabetes, and younger age were found to be risk factors for joint arthroplasty infection.