BACKGROUND More than 300 000 joint replacement operations are performed in Germany every year, and the number is rising. In this article, we consider the question of simultaneous bilateral joint replacement at the hips or knees. Such procedures are indicated in patients suffering from bilateral, symptomatic arthrosis. METHODS The article is based on a selective review of the relevant literature, and on the authors' own prospective comparative study of simultaneous bilateral hip replacement. RESULTS The morbidity and mortality of simultaneous bilateral hip arthroplasty are no different than those of a unilateral procedure. Rehabilitation is easier when both joints are replaced at the same time. Simultaneous bilateral knee arthroplasty has comparable morbidity to a unilateral procedure, but a slightly higher mortality (0.30% vs. 0.14%). Allogenic blood transfusion is more likely to be needed in a bilateral procedure, particularly of the knees. Simultaneous bilateral arthroplasty of either the hips or the knees in one surgical procedure is better than two-staged arthroplasty during a single hospital stay. CONCLUSIONS Simultaneous bilateral hip arthroplasty is safer for the patient and facilitates rehabilitation, regardless of the patient's age and ASA status (ASA, American Society of Anesthesiologists). With regard to the knees, there are two additional issues, namely the more frequent need for transfusion and somewhat higher mortality of a simultaneous bilateral procedure. Therefore, we recommend simultaneous bilateral knee arthroplasty only for patients in ASA classes 1 and 2. Simultaneous bilateral arthroplasty of either the hip or knee incurs lower costs than two separate operations.