Attention deficit/ hyperactivity disorder (ADHD) is a psychiatric disorder with unclear neurobiological background, thought to be associated with altered dopaminergic, noradrenergic and serotonergic (5-HT) systems functions. Platelet 5-HT concentration is changed in particular behaviors (aggressive, impulsive) and cognitive disturbances, related to ADHD. The study used platelets as a limited peripheral model of the central 5-HT synaptosomes, since they use, release and metabolize 5-HT similarly as central 5-HT synaptosomes. Platelet 5-HT concentration was determined in groups of children with ADHD and healthy control children, and the possible influence on sex on platelet 5-HT concentration was also evaluated. The results showed the lack of significant differences in platelet 5-HT concentration between boys and girls with ADHD and control boys and girls. No significant sex related differences in platelet 5-HT concentration were found. The findings suggest that ADHD is not associated with altered platelet 5-HT concentration. Peripheral biological marker such as platelet 5-HT is related to specific behaviors or symptoms altered in ADHD, and platelet 5-HT concentration could not be used as a peripheral biological marker of a complex diagnostic entity such as ADHD.