Static posturography may provide for an objective support to the clinical observation during the evaluation of rehabilitation and functional recovery after total hip arthroplasty (THA) or total knee arthroplasty (TKA). Unfortunately though, many of the studies focused on this topic, have so far given unclear and partially contradictory results. The aim of the present study was to identify those clinically significant Postural Parameters (PPs) likely to enable to discriminate between normal subjects and THA and TKA patients, evaluated immediately before surgery and in the first year of follow-up (f-up). Two hundred forty-nine subjects (men and women) were enrolled in the study. They were split into a Reference Group (RG) and an Experimental Group (EG), made of THA and TKA patients evaluated before surgery as well as at a sixmonth and one-year follow-ups. The test was performed with Open Eyes and Closed eyes. Four Postural Parameters (PPs) were adopted to identify statistically significant differences (SSDs): mean velocity (MV), Sway Area (SA), the root mean square of the resultant distance (RMSD), and the 95% power frequency (PF_95); the last two PPs were considered in the fore-aft (suffixed with fa) and mediallateral (suffixed with ml) directions. Correlation with Anthropometric Parameters was explored and, when found, PP values were normalized. Gender differences were also considered. The data obtained are consistent with the clinical situation of the subjects and in good agreement with literature data. All the PPs of the RG did not exhibit SSDs between male and female subjects, but some SSDs with gender were found in the EG. The PPs adopted did not appear to be influenced by false within-subject (i.e. fatigue) and between-subject variability (i.e. anthropometry or positioning) and they allow to identify differences among groups. The results obtained highlight the presence of SSDs between the RG and the EG in terms of both RMSD, indicating an increased sway, and MV, indicating an increased cost for standing, especially in the medial-lateral direction. SA was found to be most robust parameter. The PPs values registered at follow-up in the EG get closer to those registered in the RG. A worsening in terms of performance seems to occur at the six-month follow-up compared to the pre-operative session and recovery seems to be slower in TKA subjects compared to THA subjects. Manuscript received February 1, 2008 L. Quagliarella is with the Sezione di Ingegneria Biomedica, University of Bari, Bari, BA 70124 Italy (phone: +39-080-5478617; fax: +39-080-5565425; e-mail: l.quagliarella@bioingegneria.uniba.it). N. Sasanelli is with the Sezione di Ingegneria Biomedica, University of Bari, Bari, BA 70124 Italy (e-mail: n.sasanelli@bioingegneria.uniba.it). V. Monaco is with IMT Lucca Institute for Advanced Studies, Lucca, LU 55100 Italy (e-mail: v.monaco@imtlucca.it). G. Belgiovine is with the Sezione di Ingegneria Biomedica, University of Bari, Bari, BA 70124 Italy (e-mail: g.belgiovine @bioingegneria.uniba.it). B. Moretti is with the Sezione di Ortopedia, University of Bari, Bari, BA 70124 Italy (e-mail: b.moretti@ortop2.uniba.it). In the EG, there is an extremely pronounced increase in RMSDml, both in EO and in CE, compared with the RG. This important feature is considered as the single best predictor of future falling risk in the literature. Finally, PF_95fa values are higher than PF_95ml values in the THA group and even higher in the TKA group, mostly in the Closed Eyes condition. The SSDs observed between the RG and the EG do highlight an increase in both RMSD and MV, i.e. a greater energy cost to maintain posture, especially in the ML direction. The decreased PP values in T2 call for a more in-depth analysis since they seem to contradict the widespread conviction that ascribes performance worsening to fatigue, especially in THA and TKA subjects. Unfortunately it is not possible to make a direct comparison with other studies because the designs of the studies in question differ a lot. The need for a methodological harmonization is therefore once again making itself felt.