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Objective: This is study aimed to determine if serum levels of brain derived neurotrophic factor (BDNF) can be used as a biological marker to make a distinction between the depressive episodes of bipolar disorder (BD) and recurrent major depressive disorder (UD). Methods: Patients between 18-65 years of age and diagnosed with BD and UD according to DSM-IV-TR diagnostic criteria, who were admitted to the outpatient clinic of psychiatry department were enrolled to the patient arm of the study. Volunteers between 18-65 years of age, who had no physical morbidity and clinical psychopathology according to DSM-IV-TR diagnostic criteria, were enrolled to the control arm of the study. There were 24 patients (11 BD and 13 UD) and 18 healthy volunteers. Patients were required to be drug naïve for the past four weeks prior to the study enrollment. Hamilton Depression Rating Scale (HDRS) were applied to all patients. The serum levels of BDNF of all the subjects were studied. Results: There were no differences between groups in terms of sociodemographic variables. Total number depressive episodes were higher in BD group compared to UD group. There were no differences between BD and UD groups in terms of serum levels of BDNF. However, in patient group serum BDNF values were lower than the control group. Serum BDNF levels did not correlate with age, body mass index or gender in each group. Serum BDNF levels did not correlate with mean scores of HDRS or number of depressive episodes. Discussion: Considering the outcomes of the present study, serum BDNF levels did not demonstrate any significant difference between patient groups. However, serum BDNF levels were lower in patient groups compared to controls. For the time being there is no valid biological diagnostic marker for psychiatric disorders. The data of the present study is far from generating a biological marker for the distinction of depressive episodes of UD and BD. [Anadolu Psikiyatri Derg 2017; 18(2.000): 108-112]