Purpose: To investigate whether sitagliptin affects copeptin and osmolality, suggesting arginine vasopressin activation and a potential for fluid retention, compared with placebo, in patients with a recent acute coronary syndrome and newly discovered type 2 diabetes or impaired glucose tolerance. A second aim was to confirm whether copeptin correlated with insulin-like growth factor binding protein-1. Methods: Fasting blood samples were used from the BEta-cell function in Glucose abnormalities and Acute Myocardial Infarction trial, in which patients recently hospitalized due to acute coronary syndrome and with newly detected abnormal glucose tolerance were randomized to sitagliptin 100 mg once daily ( n = 34) or placebo ( n = 37). Copeptin, osmolality and insulin-like growth factor binding protein-1 were analysed at baseline and after 12 weeks. Results: Copeptin and osmolality were unaffected by sitagliptin. There was no correlation between copeptin and insulin-like growth factor binding protein-1. Conclusion: Sitagliptin therapy does not appear to be related to activation of the arginine vasopressin system.