This pilot study evaluated the safety and feasibility of a 3-month progressive treadmill aerobic exercise (TM-AEX) program for persons with Parkinson disease with gait impairment. Eight subjects underwent a treadmill stress test to determine eligibility. Of these subjects, three were referred for further cardiac evaluation and five were enrolled. In 136 TM-AEX sessions, 11 falls or near falls and 9 episodes (8 asymptomatic) of systolic blood pressure drops >20 mmHg occurred. Harness supports prevented injury from falls. TM-AEX significantly improved the subjects' total Unified Parkinson's Disease Rating Scale scores and peak ambulatory workload capacities. This study suggests that an aerobic exercise program is feasible for persons who have Parkinson disease with gait impairment; however, precautions must be taken to prevent falls. Systolic blood pressure instability during exercise points to the need for autonomic dysfunction monitoring. Our data indicate that TM-AEX may reduce symptom severity and improve fitness. Further studies are needed for a better understanding of the risks and benefits of TM-AEX in this population.