Serotonin levels in blood serum and peripheral blood platelets were investigated in patients with multiple sclerosis (MS) (71 and 60 patients, respectively) using enzyme-linked immunosorbent assay. The mean serum serotonin concentration in patients with MS (210.11; 124–334.6 ng/ml) was lower (p = 0.014) than in the control group (269.48; 120–354.6 ng/ml). The mean value of platelet serotonin was 206.1 (124–334.6) ng/109, which was significantly lower (p = 0.000) than in the control group (641.7; 611.5–712.9 ng/109). A significant decrease in this neurotransmitter—its transport and free binding—was found in patients with MS. Comparative and correlational analysis showed that the level of serotonin was negatively correlated with many characteristics of the disease (its duration, severity, severity of individual neurological syndromes), which suggests the necessity of correcting the deficiency of this neurotransmitter with drugs that can affect the activity of the serotonergic system (SSRIs). The necessity of this correction can also be justified by the data available in the literature on the effect of serotonin on immune processes, therefore the appointment of serotonergic therapy is dictated by the possibility of pathogenetic influence on the autoimmune process.