Objective: Neurotrophins promote neurogenesis and help regulate synaptic reorganization. Their dysregulation has been implicated in a number of neurologic and psychiatric disorders. Previous studies have shown decreased levels of brain-derived neurotrophic factor (BDNF) in the serum of patients with psychiatric disorders such as major depressive disorder (MDD) and conversion disorder (CD). In human patients with temporal lobe epilepsy, there is an increase in both BDNF mRNA and protein levels in surgically resected hippocampi compared to controls. One study of children with epilepsy has found normal to increased serum BDNF levels compared to controls. Serum BDNF levels have not been investigated in adult patients with epileptic seizures (ES). We hypothesized that BDNF would differentiate between ES and psychogenic nonepileptic seizures (PNES). Methods: We assessed serum BDNF immunoreactivity in 15 patients with ES, 12 patients with PNES, and 17 healthy volunteers. Serum BDNF levels were measured using an enzyme-linked immunoassay. Results: Healthy controls showed higher BDNF levels (4,289 ± 1,810 pg/mL) compared to patients with PNES (1,033 ± 435 pg/mL) (p < 0.001). However, unexpectedly, healthy controls also showed higher levels of BDNF compared to patients with ES without comorbid MDD (977 ± 565 pg/mL) (p < 0.001). Conclusions: Unlike children, adults with epilepsy appear to have decreased levels of serum BDNF. Reduced serum BDNF levels can be used to differentiate adult patients with ES or PNES from healthy controls. Further human studies are needed to better understand the pathophysiology explaining the decreased serum BDNF levels found in epilepsy and in PNES.