Previous studies report conflicting frequencies of hypertension in cohorts of patients with Williams syndrome (WS). We studied blood pressure (BP) in WS using 24-hour ambulatory BP monitoring. This technique reliably measures day- and nighttime BP in a subject's natural environment and provides better prognostic information on long-term risks of hypertension than casual BP determinations. Twenty WS subjects evaluated through a multidisciplinary WS clinic and 35 age and gender-matched controls were studied. We found that WS subjects had significantly higher ambulatory BP than controls. After controlling for age, sex, and weight, the diagnosis of WS added approximately 10 mmHg to mean daytime and nighttime BPs. Hypertension, as defined by elevated mean daytime BP, was present in 40% of WS subjects versus 14% of controls (P < 0.05); among the children studied this difference was even more dramatic with 46% of WS children versus 6% of control children classified as hypertensive (P = 0.01). We also demonstrated normal diurnal BP variation but no evidence of a "white coat" effect or increased BP variability. Interestingly, parental reporting of a history of infantile hypercalcemia was strongly associated with the presence of hypertension (P = 0.008). Our data demonstrate that both children and adults with WS have higher mean BP and higher frequency of hypertension than healthy controls. Thus, elevated BP readings in the office setting should not be dismissed but require more thorough assessment.