To better describe the phenomenon of white coat hypertension (WCH) in children, we reviewed our single-center experience using ambulatory blood pressure monitoring to determine: 1) how the choice of threshold limits for defining ambulatory hypertension affects the determination of WCH, and 2) whether the severity of casual hypertension predicts the occurrence of WCH. Using the same daytime ambulatory blood pressure (ABP) data from 71 children (age 11.9 3.4 years) with persistently elevated casual blood pressure (BP), the prevalence of WCH was compared using 95th percentile BP limits from the Task Force on High Blood Pressure in Children (TF) and from normative pediatric daytime ambulatory BP (ABP) data. To quantify casual hypertension severity, average clinic BP was divided by the patient-specific TF 95th percentile BP to generate a BP index (ie, BP index of 1.1=10% above 95th percentile). The WCH prevalence was lower by normative ABP criteria than by TF criteria (31% v 59%, P < .001), but did not vary significantly by age, gender, race, or body mass index. Logistic regression showed that higher systolic BP index (P < .001) or diastolic BP index (P < .01) was associated with a lower probability of WCH. Specifically, as systolic BP index increased from 1.0 to 1.2, the estimated probability of WCH decreased from 87% to 15%, respectively. These data suggest that the use of the lower TF limits, derived solely from resting BP measurements, may overestimate WCH prevalence in ambulatory children. In addition, these data confirm in children the finding in adults that WCH is highly prevalent when casual hypertension is borderline or mild, but uncommon when moderate or severe.