Background: Inhibition of growth factor receptor signaling leads to unintended compensatory pathway activation in preclinical models, which may impact the success of subsequent therapies. We hypothesized that increased circulating levels of receptor ligands after anti-EGFR monoclonal antibody (mAb) therapy, including VEGF ligands or EGFR family ligands, may provide insights into clinically relevant feedback mechanisms. Methods: Plasma was obtained from 96 metastatic colorectal cancer (mCRC) patients: 48 previously treated with anti-EGFR mAbs and 48 mCRC patients previously treated without prior anti-EGFR therapy who were matched for total months of prior chemotherapy and interval since last chemotherapy treatment. Plasma levels of circulating cytokines with emphasis on EGFR and VEGF ligands were determined using suspension bead multiplex assay (BioRad). Comparisons were done by the two-sided nonparametric Wilcoxon signed rank test. Multivariate linear regression analysis was done with forward selection at p<0.1. Results: By Wilcoxon signed rank test, prior anti-EGFR mAb treatment was associated with differences in EGF (3.5-fold increase, p=0.0001), epiregulin (1.6-fold increase, p=0.02), and TGF-alpha (1.4-fold increase, p=0.04). There was no significant association with amphiregulin, betacellulin, or HB-EGF. Prior anti-EGFR mAb treatment was also associated with differences in TRAIL (1.4-fold decrease, p=0.001) and with differences in VEGF-C (1.4-fold increase, p=0.01), and FGF-basic (1.8-fold increase, p=0.02). After multivariate analysis, prior anti-EGFR mAb treatment remained significantly associated with differences in EGF (p=0.003), TRAIL (p=0.045), VEGF-C (p=0.02), and FGF-basic (p=0.02). Conclusions: Prior treatment with anti-EGFR mAbs is associated with an apparent compensatory increase in EGFR ligands and pro-angiogenic factors, and lower levels of the pro-apoptotic cytokine TRAIL. These differences in circulating growth factor receptor ligands may provide insights on resistance mechanisms and optimal treatment sequencing.