There is sound clinical evidence that many benign breast conditions, especially pain, nodularity, and cysts, are likely to have their pathogenesis in hormonal events during reproductive life. Yet the classical theories of persisting hyperestrogenism and luteal progestogen deficiency have not stood up to detailed investigation. Recent work has demonstrated the importance of dynamic hormonal changes and of chronobiological rhythms--daily, menstrual, and seasonal. Recent developments and findings are reviewed to point to the directions in which the basic etiology of these conditions is likely to be discovered.