The effectiveness of an exercise training program for 52 adults with Down syndrome (M age = 39.4 years) was evaluated. The training program consisted of cardiovascular (30 minutes) and strength exercise (15 minutes) for 12 weeks, 3 days a week for 45-minutes per session. Compared to control subjects, the training group improved significantly in cardiovascular fitness and muscular strength and endurance and had a slight but significant reduction in body weight. Greater effort must be made to promote increases in physical activity participation among persons with Down syndrome and developmental disabilities in order to reduce the potential health risks associated with low fitness and sedentary behavior.