In view of the occurrence of diabetic ketoacidosis associated with the use of sodium-glucose transport protein-2 inhibitors in patients with type 1 diabetes (T1DM) and the relative absence of this complication in patients treated with liraglutide in spite of reductions in insulin doses, we investigated the effect of liraglutide on ketogenesis. Twenty-six patients with inadequately controlled T1DM were randomly divided into 2 groups of 13 patients each. After an overnight fast, patients were injected, subcutaneously, with either liraglutide 1.8 mg or with placebo. They were maintained on their basal insulin infusion and were followed up in our clinical research unit for 5 hours. The patients injected with placebo maintained their glucose and glucagon concentrations without an increase, but there was a significant increase in free fatty acids (FFA), acetoacetate and β-hydoxybutyrate concentrations. In contrast, liraglutide significantly reduced the increase in FFA, and totally prevented the increase in acetoacetate and β-hydroxybutyrate concentrations while suppressing glucagon and ghrelin concentrations. Thus, a single dose of liraglutide is acutely inhibitory to ketogenesis.