Mastalgia was experienced by a third of marathon runners and was found to be related to breast size which has previously been unreported. The link between exercise and mastalgia has yet to be established; however, this study identified that exercise was the most prevalent factor in mastalgia occurrence which may have implications for its management. The number of participants who took no measures to relieve their mastalgia, or resorted to pain medication, highlights the importance and significance of research into exercise-related mastalgia.