Background Masked hypertension (MH), a condition in which classified as normotensive by office measurement are hypertensive with ABPM, was one of those interesting patterns. Such patients have increased rates of cardiovascular morbidity, almost as high as seen in those with both clinic and ambulatory hypertension. But there is little study for clinical features of masked hypertension in the treated hypertensives. We investigated the clinical features and any predictors of MH in treated hypertensive patients. Method From Jan. to Dec. 2005, we evaluated ABPM, echocardiogram, PWV, and office BP simultaneously in treated hypertensives. Office and awake SBP > 140 mmHg were classified as hypertensive. We compared well-controlled hypertensives (group I) with MH (group II). Results Total of 505 patients had completed ABPM appropriately. 32.2% (162), 8.3% (42) comprised group I and II respectively. Baseline characteristics showed in table 1. By logistic regression analysis including age, office BP, BMI, LV mass and PWV, BP and PWV were always statistically significant for masked hypertension. By ABP data, all kinds of BPs were higher in group II significantly (table 2). Conclusion We thought that in the well-controlled hypertensives classified by office measurement, old-aged patients showing a high aortic PWV might have a MH. So recommended the ABPM in those patients and added additional antihypertensive medications.