The purpose of this study was to evaluate the effects of an aerobic training program on adults with Down syndrome. 25 male adults with Down syndrome (M age = 24.6 yr., [Qs = 45-60) participated in a 12-wk. exercise training study. Subjects were assigned to an experimental group (n = 15) and a control group (n = 10). Pre- and posttraining treadmill tests were performed to determine the following peak physiological parameters: heart rate (HR peak), peak minute ventilation (V(E) peak), peak oxygen consumption (VO2 peak, absolute and relative), and time to exhaustion (min.). The experimental group underwent a 12-wk. interval training program. The control group received no structured exercise training during this period. Following the 12-wk. training program, significant improvements (p < .05) in peak physiological parameters were seen for the experimental group for VO2 peak (both relative and absolute). V(E) peak, and time to exhaustion. No significant improvements in peak physiologic parameters were seen in the control group. Consistent with prior research using similar subjects, we concluded that adults with Down syndrome are able to improve their aerobic capacity when performing a systematic and well-designed aerobic training program.