A polymorphism in the serotonin transporter gene (5-HTTLPR) is frequently studied for association with antidepressant treatment response, different personality traits, and psychiatric disorders. Baseline platelet serotonin (5-HT) concentration has been proposed to indicate a good or a poor treatment response to antidepressant drugs and to be associated with particular symptoms in psychiatric disorders. The aim of the study was to elucidate the genotype-phenotype relationship between platelet 5-HT concentration and 5-HTTLPR in healthy subjects. The frequency of 5-HTTLPR genotypes and alleles, as well as platelet 5-HT concentration was evaluated in 434 male and 86 female unrelated healthy medication-free Caucasian subjects of Croatian origin. A two-way ANOVA revealed no significant difference in platelet 5-HT concentration subdivided according to the particular 5-HTTLPR genotype, no significant effect of sex, no significant effect of genotype, and no significant interaction between sex and genotype on platelet 5-HT concentration. In addition, one-way ANOVA did not detect significant effects of homozygous S/S genotype, or homozygous L/L genotype on platelet 5-HT concentration. Our results showed a lack of significant association between platelet 5-HT concentration and 5-HTTLPR variants, suggesting that there is no functional relationship between 5-HTTLPR alleles and platelet 5-HT concentration in the large groups of healthy male and female medication-free Caucasian subjects, free of neuro-psychiatric disorders.