This study focused on the young adult men’s perceptions and behavior toward their women partners who they acknowledged as experiencing the effects of premenstrual syndrome (PMS). A qualitative study was conducted, framed by social constructivism, where individual interviews with 20 young Brazilian men aged 21 to 29 years were analyzed thematically. Four descriptive categories to express the men’s experiences: (a) men’s observations on partner’s behavior changes, (b) early in the relationship: apprehension and confusion, (c) knowledge about PMS led men to better understanding about changes, and (d) need for support from a health care provider and medication. The men’s perceptions and behavior toward their partners were strongly influenced by biomedical conceptions of PMS. Participants believed that their partners’ emotions and behaviors were determined by PMS during some days of the month, consequently PMS had affected the couple’s relationship. Another consequence of such medicalization was that women’s complaints about PMS were rendered invisible except when viewed as a serious medical problem requiring cure, rather than a part of women’s cyclical patterns. It is the case that the systematic description of men’s perceptions about their partner’s PMS provides an approach to this topic in educational and health care activities, with the potential to improve gender relations.