Brain-derived neurotrophic factor (BDNF) is a growth factor, which has a major role in survival, growth and differentiation of neurons in central and peripheral nervous systems. Its positive influence on neurogenesis and neuronal plasticity has a big effect on learning and memory. Furthermore, BDNF was also recognised as a significant factor in the growth of axons and dendrite formation. Increasing evidence over the last three decades has indicated that the pathophysiology of many psychiatric diseases and disorders, including schizophrenia and major depressive disorder, involve brain-derived neurotrophic factor. Reduced BDNF levels have been observed in the blood of patients suffering from these diseases. Psychological disorders are a result of genetic and environmental influences, and due to their complexity they represent a big problem in society. The aim of this study is to measure and compare BDNF concentrations in serum of psychiatric patients suffering from schizophrenia and major depressive disorder, with the intention of finding an objective molecule that would serve as a biomarker, and to investigate the possible BDNF contribution to differential diagnosis of these two disorders. This study included 69 patients, of which 31 are suffering from major depressive disorder and 38 from schizophrenia, and which are treated at the Psychiatric Clinic of University Hospital Centre „Sestre milosrdnice“. Examinees differ by age and sex, but there are many more female participants. Serum BDNF concentration is determined by ELISA (Enzyme-Linked Immunosorbent Assay) method using the ready-made reagent kit, in which the reaction is based on noncompetitive sandwich principle. A computer statistical program (MedCalc Software, Mariakerke, Belgium) was used for the statistical analysis. Normal distribution of variables was tested using D'Agostino-Pearson test, that lead us to using parametric testing for data comparison. The results have shown that there is no significant difference in the serum BDNF concentration in these two groups of patients, meaning that from this point of view, measuring BDNF levels has no differential diagnostic value. Beside that, the comparisons of values between male and female examinees, patients that are on therapy (Pharm patients), patients with determined SERT polymorphism have been done, and the correlation of BDNF levels and age has also been tested. Neither one of these investigations has given us a statistically significant result. Nevertheless, it would be of interest to further explore this issue on a larger sample size and in more controlled environmental conditions.