The association of menstrual dysfunction and weekly running mileage was investigated in a group of habitual women runners, categorized into high mileage runners (30 or more miles/week) and low mileage runners. Both mileage groups reported menstrual changes and weight loss since starting to run. Menstrual irregularities (35 or more days between cycles) were reported more often among the high mileage women than among the low mileage women (p = 0.001). However, after controlling for cycle irregularity before starting a running program, the effect of mileage was no longer statistically significant. While these results suggest that pre-running menstrual irregularity is more important than higher doses of weekly mileage, further investigations are needed to assess the role of potentially confounding factors such as stress, diet, and other lifestyle changes that may occur with increased exercise.