With the main exception of Emily Martin (1992), menstruation has not played a central role in most medical anthropological studies of women’s lives. This is despite the fact that the near-universal experience of menstruation not only impacts women’s lives on a day-to-day basis, but it also has far-reaching effects on economy, politics, religion, science, and other societal realms. This paper, based on twelve months of fieldwork at a women’s university in Tokyo, highlights the meanings and lived experiences of menstruation for young Japanese women. Menstruation is not simply a biological phenomenon experienced in a culture-free vacuum. I argue that sex/health education, menstrual product design and advertising, and discourses on motherhood and femininity all influence the embodied experience of menstruation. While menstrual products are readily visible on television screens and on drugstore shelves, menstruating women carry out vigilant routines of concealing their menstrual status, creating an illusion of absence. “Putting up with it” is a round-the-clock task; even if they are not encumbered by menstrual cramps or fatigue, young women cannot ignore their flow, remaining hyper-aware of the positioning of their bodies and the integrity of their napkins. Leaks are signs of failure – breaches of the hidden world of menstruation that women are taught from childhood to meticulously guard. Studying this hidden world can help us understand more about Japanese women’s lives and embodied experiences, as well as their status and relationships with(in) the state, hygiene product businesses, and mass-produced media.