Coronary reimplantation during arterial switch operation (ASO) may affect coronary artery growth and function during childhood. The purpose of this study was to assess coronary artery diameter and regional myocardial blood flow (MBF) and myocardial flow reserve (MFR) in children after neonatal ASO. We measured proximal diameters of left anterior descending (LAD), left circumflex, and posterior descending coronary arteries on coronary angiogram in 12 children (median age 11 years, range 7.6 to 15.1) with a history of neonatal ASO. These children then underwent cardiac positron emission tomographic imaging using nitrogen-13 ammonia to assess MBF at baseline and during intravenous adenosine hyperemia in regions supplied by these 3 coronary arteries. Coronary artery z-scores were within normal range (-2.0 to 2.0) for 32 of 36 coronary arteries. MFR (ratio of hyperemic to basal MBF) was normal (>2.5) in all myocardial regions in 10 of 12 patients. The remaining 2 patients, 1 with a dual LAD and 1 with LAD origin from the right coronary artery, had generalized impairment of hyperemic MBF (<2.0 ml/g/min) and low MFR (<2.5). Coronary artery z-scores and MFR in corresponding myocardial territories were not correlated (r = 0.15, p = 0.36). In conclusion, coronary growth and function appear to be normal in most children after neonatal ASO. Children with anatomic LAD abnormalities may be at increased risk of impaired MFR.