The hippocampus is the primary site of memory and learning in the brain. Both normal aging and various disease pathologies (e.g., alcoholism, schizophrenia, and major depressive disorder) are associated with lower hippocampal volumes in humans and hippocampal atrophy predicts progression of Alzheimers disease. In animals, there is convincing evidence that exercise training increases hippocampal volume. A recent cross-sectional study in older humans demonstrated a positive relation between aerobic fitness and hippocampal volume. Moreover, a recent study found that a year of aerobic exercise training increases hippocampal volume by 2% in older adults. Accordingly, in this preliminary study the authors sought to confirm the direct effect of supervised exercise training on magnetic resonance imaging (MRI) estimates of hippocampal volume in healthy humans. The preliminary results suggest a linear relation between changes in cardiorespiratory fitness and hippocampal volume such that large gains in cardiorespiratory fitness with 10 weeks of supervised exercise training are associated with augmented right and left hippocampal volumes. (Contains 3 figures and 2 tables.)