BACKGROUND: Masked hypertension (MH) is a significant predictor of target organ damage and associated with poor cardiovascular prognosis. However, it remains to be elucidated whether MH modifies metabolic syndrome (MetS) in treated hypertensive patients. METHODS: A total of 1,086 outpatients (5710year, female; 52%) with chronically treated hypertension in university hospitals were enrolled. Patients were classified into 4 groups according to hospital (���140/90 mm Hg) and home (���135/85 mm Hg) BP levels; controlled hypertension (CH: low hospital and home BP), white-coat hypertension (WCH: high hospital but low home BP), masked hypertension (MH: low hospital but high home BP), and uncontrolled hypertension (UH: high hospital and home BP). MetS was defined by the ATP III criteria. RESULTS: Ecah prevalence of CH, MH, WCH, and UH was 38, 11, 24 and 27%. The prevalence of MetS in UH (52%) and MH (45%) were higher than those in CH (35%) and WCH (41%) (p<0.001 for trend), although the number of target organ damage was similar among groups. These differences of MetS were mostly led by the increase of abdominal circumference (86.8cm of CH or 86.5cm of WCH vs.89.4cm of MH or 88.9cm of UH, all p<0.01). Fasting glucose was significantly higher in MH compared to those of CH and WCH. However, there was no difference in HDL-cholesterol and triglyceride among groups. CONCLUSIONS: Our findings indicate that masked hypertension is associated with metabolic syndrome and abdominal obesity in treated hypertensive patients, which is similar to uncontrolled hypertension and comparable to controlled and white-coat hypertension.