Current research focusing on the biological background of premenstrual syndrome suggests that both psychological and physiological factors exert their effect by means of the dysregulation of the serotonergic function. The results of several studies point to a variation in the function of the serotonergic system throughout the cycle, which is related to the plasma concentration of the gonadal steroid hormones. Certain parameters of serotonergic function show different values in the different phases of the cycle. The rise in oestrogen concentration is followed by an increase in the 5HT2A receptor expression in brain areas responsible for mood, emotion, cognition and neuroendocrine regulation. The role of serotonin is further supported by its role in depression, anxiety, aggression as well as the efficiency of serotonergic agents in the treatment of premenstrual syndrome. In our article we review the main theories concerning the biological background of the premenstrual syndrome, and we outline in detail the role of the serotonergic system. We review the research on the relationship between serotonergic function and gonadal steroid levels as well as the latest results concerning the state- and trait-like parameters of the serotonergic system. The studies reviewed provide evidence for the relationship between the gonadal steroid function and the serotonergic system. Further research is necessary in order to determine why the cycle-dependent variation in the function of the serotonergic system leads to the development of premenstrual syndrome only in vulnerable women.