To determine the magnitude and the relationships of the difference between office and awake ambulatory blood pressures (BP) (white-coat effect) in ambulatory hypertensive patients, 64 consecutive patients referred to the ambulatory BP monitoring laboratory were studied. All subjects were evaluated prospectively by study nurse, study doctor, and ambulatory BP measurements. Order of measurements was randomized and observers were blinded to each others readings. No differences were found in the white-coat effects among study nurse (22/14 +/- 20/9 mm Hg), study doctor (27/12 +/- 20/10 mm Hg) and referring doctor (19/11 +/- 18/10 mm Hg). Similarly, female and male patients exhibited similar white-coat effects on the day of ambulatory monitoring. Older patients (> or = 65 years) displayed higher mean systolic white-coat effects than younger patients (29 +/- 18 mm Hg vs 19 +/- 19 mm Hg, P = 0.04). Multivariate analysis using the mean average systolic white-coat effect as the dependent variable and age, gender, treatment status, body mass index (BMI) and duration of hypertension as independent variables showed a significant independent role for age. In contrast, no clinical correlates of the diastolic white-coat effect were found. Older patients are more likely to display a systolic white-coat effect in the medical care environment.