There is an increasing interest in exercise and fitness in Alzheimer disease (AD) given evidence suggesting a role in the maintenance of cognitive health. There is, however, little data on the objective measure of cardiorespiratory fitness in individuals with AD. Thus, we assessed cardiorespiratory fitness in early AD and its relationship with physical activity levels, health markers, and cognitive performance in nondemented (Clinical Dementia Rating 0, n=31) and early-stage AD (Clinical Dementia Rating 0.5 and 1, n=31) participants. Cardiorespiratory fitness was assessed with maximal exercise testing to determine peak oxygen consumption (VOpeak2). Additionally, dual emission x-ray absorptiometry scanning for body composition and glucose tolerance tests were conducted. Despite reductions in physical performance and habitual physical activity levels in early AD, cardiorespiratory fitness (VOpeak2) was comparable in the 2 groups (19.8 in early AD vs. 21.2 mL/kg/min in nondemented, P=0.26). AD participants performed well on treadmill tests with similar levels of perceived exertion, maximal heart rate, and respiratory exchange ratio compared with nondemented individuals. After controlling for age and sex, VOpeak2 was associated with a beneficial glucoregulatory profile and inversely associated with percent body fat, body mass index, and triglycerides. A relationship between cognitive performance measures and VOpeak2 was not apparent. These results suggest that individuals in the early stages of AD have the capacity for maximal exercise testing and have comparable levels of cardiorespiratory fitness as nondemented individuals. Reduced physical activity associated with early AD underscores the need for further defining the role of exercise as a potential therapeutic intervention in the early stages of AD.