INTRODUCTION: The justification for ambulatory blood pressure monitoring is that blood pressure varies, and a better understanding of the causes of such vqariability will greatly improve our knowledge of its consequences. Although it is generally assumed that the average or true level of blood pressure is the main culprit causing vascular damage, it would be surprising if the variability did not also contribute. Variability of blood pressure is a compoex issue, and is the result of internal and external influences acting in various combinations.