Mood disorders are among the most common "major" psychiatric disorders. One of the more prominent among them is depression, the 4th leading health problem in the world accounting for 4.5% of the global burden of disease. A high level of importance is attributed to depression since it has a serious impact on the ability to work as well as on personal, family and social performance. Moreover, patients suffering from depression are also distinguished by a significant risk of suicide. Although clinical examination of patients is still the most important activity in diagnosing depression, numerous studies have focused on finding biological markers of depression that would assist in completing the diagnosis. The objectives of this study were to determine and compare the level of serotonin in platelets, the platelet count and the platelet parameters in healthy subjects and in patients with a depressive disorder, and then investigate the correlation between the level of platelet serotonin and platelet indicators and the clinical picture of a depressive disorder, as well as determine whether the platelet serotonin level and indicators of platelet properties changed during the therapy with serotonergic antidepressants in patients with a depressive disorder. The study involved 60 patients suffering from depression and 63 healthy subjects of both sexes, aged 18 to 55. To diagnose depression, we used the following questionnaires: MINI, CGI, HAM-D, HAM-A. The biochemical analysis of blood needed for this study was performed in the Department of Chemistry of the University Hospital Center "Sestre Milosrdnice". The research showed that the level of platelet serotonin and serotonin in whole blood and the platelet indicators did not differ between the patients suffering from depression and the control group. After an eight-week long treatment with serotonergic antidepressants, the patients suffering from depression recorded a reduction in the mean platelet volume in whole blood and a decrease in the platelet serotonin level. We also established correlation between platelet serotonin levels and levels of serotonin in whole blood and the diagnostic features indicating presence or absence of depression. On the basis of our study, we determined that the diagnostic accuracy of the level of serotonin in platelets was satisfactory given the high sensitivity of the test, as well as the positive predictive value of the relevant parameter used. We also found that it was highly important to recognize depression in patients with cardiovascular diseases, especially coronary heart diseases, because therapy with serotonergic antidepressants has an effect on the mean platelet volume in whole blood and the level of serotonin in platelets, and both of these parameters are factors in the process of platelet activation. By reducing both values, therapy with serotonergic antidepressants reduces possible complications and improves the course of recovery of these patients.