Regular physical activity is protective against, and beneficial for, mild cognitive impairment (MCI), dementia, and Alzheimer's disease. The mechanisms underlying these benefits remain unknown although it has been suggested that exercise-induced changes in the circadian pattern of cortisol secretion may be implicated. Fitness, salivary cortisol levels (0 and 30 min postawakening, midday, 5 p.m., and 9 p.m.), and cognitive function were determined in a group of amnestic MCI patients (n = 39) before and after a three-month exercise program (n = 19) or usual care (n = 20). At baseline, fitness measures were positively correlated with peak levels of cortisol and a greater fall in cortisol concentration from peak levels to midday. The exercise intervention successfully increased fitness and resulted in a greater fall in cortisol concentration from peak to midday, compared with the control group. The exercise intervention enhanced indices of executive function, although memory, mood, and functionality were not affected.