OBJECTIVE To review current knowledge on blood pressure variability. Blood pressure variability has a time-course ranging from a few seconds or minutes (short-term variability) to 24 h (long-term variability) or 1 year (seasonal variations). The variability is influenced by physiological factors such as physical and mental activity (posture, exercise, talking) or behavioral and environmental factors (salt, caffeine, alcohol), and by pathological conditions. In patients with essential hypertension, the day-night pattern of blood pressure change is generally similar to that of normotensives, with a significant nocturnal blood pressure fall (dippers), except that the entire profile is shifted upwards. Nevertheless, in some essential hypertensives the nocturnal fall in blood pressure is absent or reversed (non-dippers), in spite of a decrease in the nocturnal heart rate. In several forms of secondary hypertension (pheochromocytoma, renal failure) and other clinical conditions (sleep apnea syndrome, diabetes mellitus, cardiac transplantation) the nocturnal fall in blood pressure is also absent or reversed. CONCLUSIONS Blood pressure variability and the blunted nocturnal fall in blood pressure may be clinically relevant. Several studies have demonstrated that subjects whose 24-h variability was higher than the group average were more likely to have target-organ damage. Moreover, hypertensive women with a blunted nocturnal fall in blood pressure (non-dippers) are more likely to suffer morbid cardiovascular events than dippers. On theoretical grounds, therefore, antihypertensive treatment that reduces blood pressure variability and preserves the nocturnal fall in blood pressure will help to protect target organs in hypertension. So far, this has not been demonstrated in clinical trials.