People with an intellectual disability are less physically active, live more sedentary lives, have lower fitness levels and are more likely to be overweight or obese than the general population. No evidence exists on the impact of participation in Special Olympics Ireland (SOI) on physical activity and physical fitness levels. Adults with intellectual disabilities (16-64 years) were recruited from services and SOI clubs. Physical measures included waist circumference, height, weight, blood pressure, heart rate and 6-min walking test. Self-report questionnaires gathered data on physical activity levels. Actigraph (GT3X) accelerometers were used to gain an objective measure of physical activity. SOI participants accumulated more moderate to vigorous physical activity per day, had higher fitness levels and more positive health profile scores than those not taking part in SOI. SOI has the potential to make a positive difference to people's physical health and subsequently their overall health and well-being.