Abstract--Little is known about the clinical significance of isolated ambulatory hypertension, a condition characterized by low office but elevated ambulatory blood pressure. This study aimed to investigate the prevalence and the predictive value of isolated ambulatory hypertension diagnosed after 3 months of observation for the development of sustained hypertension within a cohort of 871 never-treated stage-1 hypertensive subjects. The study end point was progression to more severe hypertension and need of antihypertensive medication. In 244 subjects (28%), clinic blood pressure declined to <140/90 mm Hg after 3 months. Of these, 124 (14.2% of total) had low clinic and ambulatory blood pressures after 3 months (nonhypertensive subjects), whereas 120 subjects (13.8% of total) showed low clinic but elevated ambulatory blood pressure (isolated ambulatory hypertension). During the 6 years of observation, the number of end points based on multiple clinic blood pressure readings progressively increased from the nonhypertensive subjects (19%) to the subjects with isolated ambulatory hypertension (35%) and to the subjects with high clinic and high ambulatory blood pressures (65%, P<0.0001). In an adjusted proportional hazard model, isolated ambulatory hypertension status was associated with a 2.2 (P=0.02) increase in the risk of reaching the end point in comparison with the nonhypertensive subjects. Final ambulatory systolic blood pressure was also higher in the former than the latter (P=0.03). Our results indicate that among subjects screened for stage 1 hypertension, individuals with isolated ambulatory hypertension after 3 months of observation have increased risk of developing sustained hypertension in later life compared with subjects in whom both clinic and ambulatory blood pressures are normal.