Hip or knee arthroplasty healthcare-associated infections (HAI) are a public health problem that induces the increase of morbidity and mortality rates and poses an economic problem with significant impact on hospitals budget. The infection rate in primary hip and knee arthroplasties range between 1.5% and 2.5%, and is considered one of the main reasons for surgeries non-effectiveness. A retrospective study was carried out in S. João Hospital Center, EPE (CHSJ) to calculate HAI rate in primary hip and knee arthroplasties, and to analyse their direct costs, for a better understanding of their economic impact. Four hundred and eighty seven arthroplasties were studied and infection was noticed in 11 cases: 3 after hip and 8 after knee arthroplasties. Data collected from infected patients-related costs were compared with the average cost of non-infected patients (standard). An incidence rate of 2.17% for hip arthroplasties and 2.25% for knee arthroplasties was found. Results showed that patients with infection remained in hospital 7.45 times longer than uninfected patients and incurred hospital costs almost 3.8 times higher. This work shows how important is the quantification of additional HAI costs to allow hospital managers to weigh the cost/benefit ratio and better justify investments in HAI prevention and control programmes.