PURPOSE: Obesity, insulin resistance, and puberty seem to influence and been inversely associated with 25-hydroxy vitamin D (25OHD) levels. To our knowledge, a study on 25OHD in children and adolescents with Prader-Willi syndrome (PWS), a genetic form of obesity, is not yet available.OBJECTIVE: To analyze the 25OHD values in pediatric PWS subjects in comparison with a control group (CNT), highlighting the possible correlations with IR, BMD, body composition, pubertal stage, and GH therapy (GHT).METHODS: Auxological and laboratory parameters, HOMA-IR, Vitamin D status, and bone density and body composition by DEXA scan were analyzed in 52 PWS and 110 controls (CNT), gender-, age-, and BMI-SD matched. None of them was on calcium or vitamin D. 20 PWS were on growth hormone (GH) therapy and 32 were previously treated.RESULTS AND CONCLUSION: Altogether, PWS had similar values of 25OHD compared to CNT.16 PWS (30.7%) and 27 CNT (24.5%) had low 25OHD levels (<20 ng/ml) (p = NS). 25OHD of PWS on GHT were comparable to those previously treated. In both groups, univariate analysis showed a negative correlation between 25OHD and fat mass% (FM%). GH therapy and pubertal stage were positively correlated with bone parameters analyzed by DXA. Multivariate regression confirmed only FM% as negative predictor of 25HOD in PWS patients, as previously described. GHT does not seem to influence 25OHD in PWS.CONCLUSION: Our data showed that PWS had similar values of 25OHD compared to CNT. As already described, FM seems to be the only parameter influencing 25OHD levels. Finally, GHT does not seem to influence 25OHD metabolism in PWS.