Ambulatory blood pressure recordings were performed in patients receiving anti-hypertensive therapy to determine the prevalence of a white coat effect and to evaluate the importance of the sex of the patients with higher office vs. ambulatory blood pressure (BP) readings. A white coat effect (office--ambulatory BP > or = 20/10 mm Hg) was present in 106 of 152 treated hypertensive patients who had a mean office BP (mm Hg) of 174 +/- 2/94 +/- 1 compared with a mean awake ambulatory BP of 137 +/- 2/81 +/- 1 (P < 0.001). A severe white coat effect (office--ambulatory BP > or = 40/20 mm Hg) was observed in 49 of 152 patients with office BP (183 +/- 3/96 +/- 2) being higher (P < 0.001) than mean awake ambulatory BP (134 +/- 2/77 +/- 1). A white coat effect was present in 70 of 87 women but only in 36 of 65 men (P < 0.001). Similarly, a severe white coat effect was seen in 41 of 87 women but only in 8 of 65 men (P < 0.001). Female sex and office SBP were significantly (P < 0.001) correlated with the presence of a white coat or severe white coat effect. Thus, female patients with treated hypertension are more likely than men to have a white coat component to their high office BP readings. These sex differences in white coat effect should be considered in future studies involving patients with hypertension based on high office readings.