Vitamin D deficiency seems to predict the unsuccessful achievement of sustained viral response (SVR) after anti-viral treatment in hepatitis C virus (HCV) difficult to treat genotypes. Vitamin D binding protein (GC) gene polymorphisms are known to influence vitamin D levels. This study was performed to assess whether the interaction between basal circulating vitamin D and the GC polymorphism plays a role in influencing the rate of anti viral responses in patients affected by chronic hepatitis C. Two hundred six HCV patients treated with a combination therapy of PEGinterferon plus ribavirin were retrospectively evaluated. GC rs7041 G>T, GC rs4588 C>A and IL- 28B rs12979860 C>T polymorphisms were genotyped. Frequencies of GC rs7041 G>T and rs4588 C>A polymorphisms were: G/G=64 (31.1%), G/T=100 (48.5%), T/T=42 (20.4%) and C/C=108 (52.4%), C/A=84 (40.8%), A/A=14 (6.8%). Patients were divided into those carrying ≥3 major alleles (WT+: G-C/G-C, G-C/T-C, G-C/G-A, N=100) and the remaining (WT-: G-C/T-A, T-A/T-C, T-A/T-A, T-C/T-C, N=106). Four groups were identified: vitamin D≤20 ng/mL and WT-, vitamin D≤20 and WT+, vitamin D>20 and WT-, vitamin D>20 and WT+. In difficult to treat HCV genotypes the proportion of patients achieving SVR significantly increased with a linear trend from the first to the last group: 6/25 (24.0%), 9/24 (37.5%), 12/29 (41.4%), 19/29 (65.5%) (p=0.003). At multivariate analysis having basal vitamin D >20 ng/mL plus the carriage of GC WT+ was found to be an independent predictor of SVR (O.R. 4.52, p=0.015). Conclusions: in difficult to treat HCV genotypes, simultaneous pre treatment normal serum vitamin D levels and the carriage of GCglobulin wild type isoform strongly predicts the achievement of SVR after PEG-interferon plus ribavirin antiviral therapy. Page 3 of 28 Hepatology