Purpose. To determine the effect of an aquatic aerobic exercise program on body structure and function, activity and participation in a five year-old girl with a diagnosis of spastic-diplegic CP. Method. A single subject A1-B-A2 design was used. The following outcomes were measured: Energy Expenditure Index, 6 Minute Walk Test, Gross Motor Function Measure, and the Canadian Occupational Performance Measure. Intervention . The aquatic aerobic exercise was conducted 3 x /week for 12 weeks. Each session included 30-40 minutes of aerobic training at an intensity of 50 to 80% of maximum heart rate. Data Analyses were conducted using visual analysis, Two Standard Deviation Band Method and Statistical Control Process theory. Results. Statistically significant improvements from the baseline to the end of intervention phase were found for all outcome variables. Conclusion. A 2-3 times per week aquatic aerobic exercise program may improve endurance, function and participation in children with spastic CP. Continued study with larger sample-size is recommended.