A recent study by Aviles-Olmos and colleagues suggests that 12 months of treatment with the glucagon-like peptide-1 receptor agonist exenatide improves motor and cognitive symptoms in Parkinson's disease (PD), and that the effect persists as long as 12 months after termination of the treatment. Due to the lack of a placebo control, one cannot exclude that the observed differences between patients receiving daily injections of exenatide and non-treated controls are due to a placebo effect. We discuss that large group differences in two independent functional measures remain for at least 12 months following the cessation of exenatide treatment and that this warrants a double-blind placebo-controlled trial with exenatide in PD.