Prader-Willi syndrome (PWS) and Ehlers-Danlos syndrome (EDS) are two different genetical disorders both characterized, among other features, by muscular hypotonia. Postural control seems to be impaired in both conditions. The aim of the present study was to quantitatively compare postural control in adult PWS and EDS using stabilometric platform to unveil possible common determinants of impaired balance. We enrolled 11 PWS and 21 EDS adult patients and 20 age-matched controls. They were instructed to maintain an upright standing position for 30s with open eyes (OEs) focusing on a 6 cm black circle positioned at a distance of 1.5m. Both PWS and EDS patients were characterized by higher RANGEML, RANGEAP and trajectory length of CoP values as compared to CG. No statistically differences were found between PWS and EDS in terms of any of these parameters. The results demonstrated that both PWS and EDS are characterized by a severe postural instability. Muscle hypotonia and weakness may account for reduced balance capacity. Quantitative characterization of instability is important to identify, develop and enhance rehabilitation interventions.