Relations between physical activity and prospectively collected menstrual cycle characteristics were examined in two large cohorts. One cohort consisted of women employed in the semiconductor industry in 1989 who participated in a prospective study of reproductive outcomes (n = 367). The other consisted of women living in Tecumseh, Michigan, who completed both the 1992-1993 and 1993-1994 examinations for the Michigan Bone Health Study (n = 328). Mean cycle length, variability of cycle length, and mean bleed length were calculated from daily diaries (Semiconductor cohort) or monthly menstrual calendars (Michigan cohort) for a median of five and 11 cycles, respectively. Physical activity was assessed by self-report at baseline and expressed as metabolic equivalent-minutes per week. In the Semiconductor study, women also reported daily minutes of vigorous exercise in their diaries. In the Michigan cohort, total physical activity, total recreational physical activity, and vigorous recreational activity were positively associated with cycle length. The magnitude of these associations declined as body mass index increased. In the Semiconductor cohort, the minutes of daily vigorous exercise were positively associated with cycle length only in a repeated-measures analysis. These findings lend modest support to the hypothesis that moderate levels of physical activity can lengthen the menstrual cycle.