AbstractTo assess the influence of environmental conditions on the pattern of menstrual variation among women, data on menstrual cycles were recorded longitudinally from 127 university women who lived in a group of houses surrounding a central courtyard. Adequate follow‐up was available on 76 women, among whom menstrual synchrony occurred in a short period of time. During the first month of coresidence, the day of menstrual onset deviated from the mean by an average of 13.7 days. During the next month, however, this average deviation declined to 2.6days. Moreover, it was found that menstrual synchrony occurred in the overall sample regardless of the house of residence, although synchrony occurred to a greater extent in some houses than in others. It would thus appear that common environmental influences play an important role in the overall pattern of menstrual cyclicit