Masked hypertension is defined as a normal blood pressure (BP) in the clinic or office (135/85 mmHg). It may occur in as many as 10% of the general population, and is important because it is not diagnosed by routine medical examinations, but carries an adverse prognosis, both in terms of increased target organ damage and cardiovascular events. Possible characteristics of individuals with masked hypertension are: relatively young age, male sex, stress or increased physical activity during the daytime, and smoking or drinking habits. Masked hypertension has also been described in treated hypertensive patients (in whom the prognosis is worse than predicted from the clinic pressure) and in children, in whom it may be a precursor of sustained hypertension. It may be suspected in individuals who have a history of occasional high BP readings, but who are apparently normotensive when checked in the office. One practical point is that we should continue to follow such people rather than dismissing them, and encourage out-of-clinic monitoring of BP. This would apply particularly to smokers and those with BP in the prehypertensive range. The potential implications of masked hypertension are huge, but the optimal strategy for detecting the condition in the general population is not yet clear.