The risk and factors related to the development of hypertension among healthy youths with elevated ambulatory and normal conventional blood pressure, masked hypertension, have not been established. We performed a long-term follow-up study assessing how hypertension develops over time in healthy, masked hypertensive youths. The potential sex dimorphism in the incidence and timing of the development of hypertension has been analyzed. In a long-term follow-up study (median follow-up, 36 months), we enrolled 272 healthy conventional normotensive youths (aged 6-18 years; 55.8% girls) of whom 39 had masked hypertension at baseline. Development of sustained hypertension (hypertension in both conventional and ambulatory measurement) was recorded. The daytime systolic blood pressure increased from baseline to last available follow-up in boys (3.5 mm Hg; P<0.001) but not in girls (0.7 mm Hg; P=0.23), leading to a significant between-sex difference (P=0.0022). The incidence of sustained hypertension was 7.0/100 subjects/y (n=12) in masked hypertensives and 0.6/100 subjects/y (n=4) in normotensives. Masked hypertensive boys more frequently proceeded to sustained hypertension as compared with masked hypertensive girls (50.0% versus 17.4%; P=0.041). Masked hypertension at baseline (hazard ratio, 15.6; 95% confidence interval, 4.91-49.7; P<0.0001) and male sex (hazard ratio, 3.25; 95% confidence interval, 1.12-9.39; P=0.0295) were independent factors associated with the incidence of sustained hypertension during the follow-up. In youth, masked hypertension is a precursor of sustained hypertension. The risk of developing sustained hypertension is higher in boys than it is for girls. The fact that masked hypertension is not prognostically innocent increases the importance of the diagnosis at an early age.