Children with developmental disabilities lack sufficient physical activity in their daily lives. The purpose of this study was to see if children with developmental disabilities who participated in stair-climbing exercises had better physical performance and health status than children with developmental disabilities who did not. This study adopted a pre-and-post-test experimental design, enrolling 30 subjects (experimental group, 15; control group, 15) with developmental disabilities aged 5-7 years from pediatric rehabilitation treatment departments of two regional teaching hospitals in Taiwan. The experimental group received stair-climbing exercises and physical therapy twice a week for eight consecutive weeks. The primary outcomes were health-related physical fitness assessment items, including body composition (body mass index: BMI), cardiorespiratory fitness (stair-climbing test), muscular fitness (knee-bend sit-ups), and flexibility (Chair Sit and Reach Test). The post-test measurements of BMI, cardiorespiratory fitness, sit-ups, and muscular flexibility of children in the experimental group were significantly better than the pre-test measurements. The stair-climbing exercise training results for cardiorespiratory fitness, muscular fitness, and muscular flexibility in the experimental group were significantly better than those in the control group. Stair-climbing exercise improves the physical fitness of children with developmental disabilities. The results of this study suggest that children with developmental disabilities should be encouraged to engage in this convenient and easy stair-climbing exercise to improve physical performance and health status.