Historically difficult to define, stress is, in one sense, the factor that stressors have in common in their impact on the body. Menstrual function is disrupted by stressors that activate the hypothalamic-pituitary-adrenal (HPA) axis; this activation is part of a catabolic response of the whole body that mobilizes metabolic fuels to meet energy demand. Functional menstrual disorders are associated with an increase in cortisol and with a broad spectrum of other symptoms of energy deficiency. Recent experiments suggest that exercise and other stressors have no disruptive effect on reproductive function beyond the impact of their energy cost on energy availability. These studies suggest that treatments for functional menstrual disorders should aim at dietary reform and that stress is simply low energy availability. Future experiments should carefully test this hypothesis.