Simultaneous arthroplasties are increasingly being performed during one single anaesthetic event. No national nosocomial surveillance systems have yet reported data on this issue. We compared patient populations undergoing bi- and unilateral total hip (THA) and total knee (TKA) arthroplasties in terms of two outcome variables, deep surgical site infections (SSI) and mortality, by analysing surveillance data from the Finnish Hospital Infection Programme (SIRO). A total of 8201 patients underwent 9831 total arthroplasties during 2001-2004. Of the prosthetic joints, 7.2% were inserted in a bilateral operation (range by hospital, 0.6-19.2%; range by procedure type, 5.2-9.9%). Patients who underwent bilateral operations were younger; more often males, and their ASA score was lower than those who underwent unilateral procedures. The rate of deep SSI in bi- and unilateral THAs and in bi- and unilateral TKAs was 0, 0.5, 1.0 and 0.9%, respectively. Following bilateral operations, four deep SSIs were detected, all from bilateral TKAs, three of which were on the second operative side. In these three cases, single doses of antimicrobial prophylaxis were administered 115, 155 and 218 min before incision (median time in unilateral operations: 47 min). According to multi-variate analysis, bilateral operations were not an independent risk factor for deep SSIs. Mortality did not differ between bi- and unilateral THAs or TKAs. Our surveillance data indicate that simultaneous bilateral surgery did not increase the risk of deep SSIs or death after THA and TKA. Bilateral operations may, however, require specific guidelines regarding antimicrobial prophylaxis.