The etiological diagnosis of the loosening of a total joint replacement is of highest importance for the prognosis of the patient and the survival of the revision. However no test proved to be definitive and reliable in making a diagnosis of a periprostetic infection or to exclude it. In this study a twofold comparison of the following generally accepted methods of periprostetic infection diagnosis was made: First, the results of intraoperative and extraoperative incubation of cultures among each other and secondly, the cultures with the preoperative levels of the two acute phase reactants CRP and ESR were compared. Within the prospective study started in the Istanbul University, Cerrahpasa Faculty of Medicine, Department of Orthopaedics and Traumatology in November 2001, CRP and ESR analyses as well as specimens of liquid, swab and biopsy cultures of 32 cases were examined until June 2002. In each case two specimens were taken intraoperative, which were incubated immediately in the operating room and afterwards in the laboratory. The statistical analyses were performed with the software SPSS for Windows, where the Mann-Whitney U test and the chi square test were applied. We had 24 female and 8 male cases. The average age was 61.44. Eight patients were found infected, the 24 other cases were aseptic. CRP and ESR, which are the most useful preoperative acute phase reactants, were found to have a sensitivity of 53.8% and 42.1% and a specificity of 94.4% and 100%, respectively. CRP and ESR measured together had a sensitivity of 58.3% and a specificity of 100%. It was concluded that the CRP levels in the infected cases were not significantly affected by an additional rheumatological disease of the patient. Three different kinds of microorganisms were found in the eight infected cases: methicillin sensitive coagulase (−) staphylococcus in four cases, methicillin sensitive staphylococcus aureus in three cases and methicillin resistant coagulase (−) staphylococcus in one case. For the specimens incubated in the operating room the reproduction and diagnostic power of the specimens isolated from the liquid was found significantly higher than the specimens isolated from the swap and the biopsy (p< 0.001). CRP and ESR are parameters helping the surgeons to make reliable preoperative decisions. As the diagnostic power of a normal level CRP and ESR combination is significantly different than a high level combination, we find it useful to require both tests before the operation. For the diagnosis and tracking of patients with additional rheumatological disease CRP should be required primarily. If the surgeon suspects any infection during the operation, first of all liquid specimens should be taken and tried to incubate them in the operation room conditions.