Physical activity tends to be lower in school-age children with congenital heart disease than in healthy controls. To the best of our knowledge, objectively measured physical activity levels of preschool-age children with congenital heart disease have not been studied. Methods: A total of 10 children with either coarctation of the aorta (n=6; age 3.8±0.9) or tetralogy of Fallot (n=4, age 4.3±0.9) were recruited from the cardiology unit of McMaster Children’s Hospital. Height (103.7±8.2 cm) and weight (17.3±2.7 kg) measurements were recorded, and physical activity was determined using accelerometry over 7 consecutive days. Patients were compared with age-, sex-, and season of data acquisition-matched controls. Parents completed a questionnaire regarding the child’s physical activity and sedentary behaviours. Results: Patients spent on average 219.4±39.9 minutes engaged in total physical activity per day at the following intensities: light, 147.5±22.3; moderate, 44.0±11.8; moderate-to-vigorous, 71.9±22.6; and vigorous, 27.9±11.7. No significant differences were observed between patients and controls for total physical activity (p=0.80) or any of the intensities (p=0.71, 0.46, 0.43, and 0.45, respectively). Only 40% of patients and controls met the new Canadian Physical Activity Guidelines for the Early Years of at least 180 minutes of physical activity at any intensity every day. Of the patients’ parents, 90% believed that their child was as active, if not more active, than his/her siblings, and 80% of parents reported their child spending 1–3 hours in screen time activities daily. Conclusion: Children aged 3–5 years old with congenital heart disease have comparable physical activity levels to age-, sex-, and season-matched controls, and many do not meet Canadian Physical Activity Guidelines.