Background. Evidence suggests that endothelial dysfunction in the early postoperative period promotes myocardial injury after non-cardiac surgery. The aim of this study was to investigate the impact of colon cancer surgery on endothelial function and the association with the l-arginine-nitric oxide pathway postoperatively. Methods. Patients undergoing elective colon cancer surgery (n = 31) were included in this prospective observational cohort study. Endothelial function, as measured using the reactive hyperaemia index (RHI), was assessed non-invasively using digital pulse tonometry. RHI and plasma concentrations of L-arginine, asymmetric dimethylarginine (ADMA), dihydrobiopterin and biopterin metabolites, tetrahydrobiopterin (BH4) and total biopterin were measured before surgery, at four h after surgery and at postoperative day one and two. Cardiac troponin I was measured before surgery and once daily on postoperative days one to four. Results. Preoperative RHI was 1.86 (1.64 – 2.11) and decreased significantly during the observation period (linear mixed effects model of serial measurements, P = 0.015). Both L-arginine (P < 0.001) and ADMA (P = 0.024) decreased during the postoperative period. All biopterin metabolites were significantly decreased after surgery. A significant positive correlation was found between logAUC(l-arginine/ADMA) and logAUC(RHI) (P = 0.015) and between logAUC(L-arginine/ADMA) and logAUC(BH4) (P = 0.015). None of the patients had cardiac troponin I elevations. Conclusions. RHI was attenuated in the first days after colon cancer surgery indicating acute endothelial dysfunction. Endothelial dysfunction correlated with disturbances in the L-arginine – nitric oxide pathway. Our findings provide a rationale for investigating the hypothesized association between acute endothelial dysfunction and cardiovascular complications after non-cardiac surgery. Clinical trial registration. NCT02344771.