Masked hypertension, a high ambulatory blood pressure (ABP) in the presence of normal office blood pressure (BP), is recognized as a risk factor for cardiovascular complications in the adult population. We evaluated the prevalence of masked hypertension in pediatric patients. We studied 136 patients (59 boys and 77 girls, aged 6-25 years, mean 13.1+/-4.7 years). In all patients, office BP measurements with auscultatory technique were less than the 95th percentile for sex and age or <140/90 mmHg for those over 18 years. Masked hypertension was diagnosed when either systolic or diastolic daytime ABP values were equal to or greater than the 95th percentile for sex and height of reference values or > or =135 mmHg systolic or 85 mmHg diastolic BP for those over 15 years. Among 136 patients, 15 (11%) had masked hypertension. The prevalence of masked hypertension was higher in boys (19%) than in girls (5%), but not different between younger (< or =15 years) and older (>15 years) patients (11% vs. 12%). The diagnoses in the group with masked hypertension included 3 patients with diabetic nephropathy, 2 with obesity, and 2 with orthostatic dysregulation. In conclusion, masked hypertension is present in pediatric patients, and is more common in boys. Further study is needed to identify patients who may benefit from recognition of masked hypertension.