BACKGROUND: A hypertensive response to exercise (HRE; defined as normal clinic blood pressure (BP) and exercise systolic BP (SBP) ≥210 mm Hg in men or ≥190 mm Hg in women, or diastolic BP (DBP) ≥105 mm Hg) independently predicts mortality. The mechanisms remain unclear but may be related to masked hypertension. This study aimed to assess the prevalence of masked hypertension and its association with cardiovascular risk factors, including left ventricular (LV) mass, in patients with a HRE. METHODS: Comprehensive clinical and echocardiographic evaluation (including central BP, aortic pulse wave velocity by tonometry) and 24-h ambulatory BP monitoring (ABPM) were performed in 72 untreated patients with HRE (aged 54 ± 9 years; 60% male; free from coronary artery disease confirmed by exercise stress echocardiography). Masked hypertension was defined according to guidelines as daytime ABPM ≥135/85 mm Hg and clinic BP <140/90 mm Hg. RESULTS: Masked hypertension was present in 42 patients (58%). These patients had higher LV mass index (41.5 ± 8.7 g/m(2.7) vs. 35.9 ± 8.5 g/m(2.7); P = 0.01), LV relative wall thickness (RWT; 0.42 ± 0.09 vs. 0.37 ± 0.06; P = 0.004) and exercise SBP (222 ± 17 mm Hg vs. 212 ± 14 mm Hg; P = 0.01), but no significant difference in aortic pulse wave velocity or central pulse pressure (P > 0.05 for both). The strongest independent determinant of LV mass index was the presence of masked hypertension (unstandardized β = 5.6; P = 0.007), which was also independently related to LV RWT (unstandardized β = 0.04; P = 0.03). CONCLUSIONS: Masked hypertension is highly prevalent in HRE patients with a normal resting office BP and is associated with increased LV mass index and RWT. Clinicians should consider measuring ABPM or home BP in HRE patients.