Ambulatory blood pressure (BP) monitoring is a useful tool aiding diagnostic and management decisions in patients with hypertension. Diurnal BP variation or circadian rhythm adds prognostic value to the absolute BP elevation. The Spanish ABPM Registry has collected information from >30 000 treated hypertensive patients attended by either primary care physicians or referral specialists. The analysis of BP diurnal variation has allowed the conclusion that nocturnal BP decline is related to the level of risk. Patients with blunted nocturnal dip frequently belong to high- or very high-risk categories and specifically are often older, obese, diabetics or with overt cardiovascular or renal disease. With respect to treatment, the non-dipper profile is more often observed in patients receiving several antihypertensive drug agents, but it does not correlate with the time of drug administration. Among patients receiving only one drug, non-dihydropyridine calcium channel blockers and alpha-blockers are associated with less nocturnal BP decline than other antihypertensive drug classes, even after adjusting for the level of risk.