AIM: To identify suitable biomarkers of response to bevacizumab(BV)- it remains an open question. The measurement of serum vascular endothelial growth factor(VEGF) has been proposed as a predictive factor for this drug, even if literature data are contradictory. METHODS: We prospectively evaluated the role of BV, total and not BV-bound VEGF and angiopoietin-2(Ang-2) serum levels as potential predictive factors of response for BV in combination with an oxaliplatinbased chemotherapy. BV, Ang-2, total and not BVbound VEGF levels were measured at baseline, before 2~(nd) and 5~(th) cycle of oxaliplatin-based chemotherapy in 20 consecutive metastatic colorectal cancer patients. RESULTS: Results were correlated to response to treatment. Variability in BV levels have been found, with decreased level in less responding patients. In particular, the concentration of BV increased of 3.96 ± 0.69 folds in serum of responsive patients after 3 more cycles of therapy compared to those with stable or progressive disease with a 0.72 ± 0.25 and 2.10 ± 0.13 fold increase, respectively. The determination of free and total VEGF demonstrated that the ratio between the two values, evaluated immediately before the 2~(nd) and the 5~(th) cycle of therapy, decreased from 26.65% ± 1.33% to 15.50% ± 3.47% in responsive patients and from 53.41% ± 4.75 to 34.95% ± 2.88% in those with stable disease. Conversely, in those with progression of disease, the ratio showed the opposite behavior coming up from 25.99% ± 5.23% to 51.71% ± 5.28%. The Ang-2 levels did not show any relationship. CONCLUSION: Our data show that the ratio of not BV-bound VEGF to total VEGF serum and BV plasma concentrations for predicting the response to BV plus oxaliplatin-based chemotherapy could be a promising biomarker of response to BV. AIM: To identify suitable biomarkers of response to bevacizumab (BV) - it remains an open question. The measurement of serum vascular endothelial growth factor (VEGF) has been proposed as a predictive factor for this drug, even if literature data are contradictory. METHODS : We prospectively evaluated the role of BV, total and not BV-bound VEGF and angiopoietin-2 (Ang-2) serum levels as potential predictive factors of response for BV in combination with an oxaliplatin based chemotherapy. BV, Ang-2, total and not BVbound VEGF levels were measured at baseline, before 2 ~ (nd) and 5 ~ (th) cycles of oxaliplatin-based chemotherapy in 20 consecutive metastatic colorectal cancer patients. RESULTS: Results were correlated to response to treatment. Variability in BV levels have been found, with decreased level in less responding patients. In particular, the concentration of BV increased of 3.96 ± 0.69 folds in serum of responsive patients after 3 more cycles of therapy compared to those with stable or p The determination of free and total VEGF demonstrated that the ratio between the two values, as immediately before the 2 ~ (nd) and the 5 ~ (th) cycle of therapy, decreased from 26.65% ± 1.33% to 15.50% ± 3.47% in responsive patients and from 53.41% ± 4.75 to 34.95% ± 2.88% in those with stable disease. Conversely, in those with progression of disease, the ratio showed the opposite behavior coming up from 25.99% ± 5.23% to 51.71% ± 5.28%. CONCLUSION: Our data show that the ratio of not BV-bound VEGF to total VEGF serum and BV plasma concentrations for predicting the response to BV plus oxaliplatin-based chemotherapy could be promising biomarker of response to BV.