This study examined menstrual synchrony among roommates residing in 20 university housing units. Menstrual synchrony was examined for roommates who were close friends and roommates who were not close friends, and by housing unit. The impact of personality, social-interaction and menstrual-related factors was also studied. To avoid limitations found in previous studies, (i) data were collected prospectively and (ii) over the entire academic year; (iii) high participation rate was obtained (86%); and (iv) women who used oral contraceptives were excluded. Menstrual synchrony was found among roommates who were close friends. Synchrony was not found for roommates who were not close friends, nor by housing units. The phenomenon of menstrual synchrony may more likely occur among close friends and women with intensive social contact than under the conditions common to university dormitories.