Premenstrual syndrome (PMS) frequently occurs in women of childbearing age. There are different case definitions of PMS, one proposed by the American College of Obstetricians and Gynecologists (ACOG) and another based on the Daily Record of Severity of Problems (DRSP) scores. Here we review our recent papers indicating that the discovery of biomarkers of menstrual cycle-related symptoms is strongly dependent on the case definitions used and that the gold standard methods used to asses PMS, including the ACOG case definition, induce a high degree of false-negative findings. We propose a new case definition of the menstrual cycle-associated syndrome (MCAS), which is characterized by increased DRSP scores during the menstrual cycle and additionally by an exaggerated increase in symptoms the week prior to the menses. This case definition performed well and was externally validated by diverse biomarkers including plasma levels of progesterone and estradiol, chemokines (e.g. CCL2, CCL5 and CCL11), epidermal growth factor, hydroperoxides, paraoxonase 1 activity and complement C4. In conclusion, when evaluating menstrual cycle-related symptoms and their associations with biomarkers, we propose to assess daily measurements of the DRSP and based on those scores to a) use the diagnosis of MCAS as an indicant of menstrual cycle-related symptoms; and b) examine the associations of the time series in the DRSP and its subdomains (e.g. depression, physio-somatic, anxiety) and those in biomarkers including distributed lag models.