Alcohol use disorder (AUD) is a psychiatric disorder characterized by excessive and uncontrolled drinking that causes distress and has damaging consequences for men and women of all ages. It is one of the four most disabling diseases and it affects approximately 14.6 million persons in Europe. Objective of this study is to investigate changes in platelet serotonin concentration after four weeks of alcohol abstinence in regards to the genotype of the serotonin transporter. A total of 154 patients with AUD were included in the study. Platelet serotonin concentrations were assessed by enzyme-linked immunosorbent assay. Genotype of serotonin transporter promoter polymorphism was determined by the polymerase chain reaction-based method. We did not establish a statistically significant main effect of serotonin transporter polymorphism on platelet serotonin concentration after four weeks of abstinence. Aforementioned finding is in line with previous research suggesting a complex relationship between serotonin transporter gene and platelet serotonin levels, and congruent with the well-established genotype interaction with numerous other factors, such as sex, ethnicity, education level, and stressful life events.