Objectives The role of ambulatory blood pressure monitoring (ABPM) in the elderly is unclear. This study has examined differences between clinic and ambulatory blood pressure (BP) in a large cohort of older hypertensives, with particular respect to the factors influencing the direction and magnitude of this difference.Design The Second Australian National Blood Pressure Study (ANBP2) is a general practice-based randomized-outcome trial in 6083 older hypertensives treated with an angiotensin-converting enzyme (ACE) inhibitor or diuretic-based regimen. Before starting treatment a subset of 713 patients (age range 65-83 years) had a 'successful' 26-hour ambulatory blood pressure recording with a SpaceLabs 90207 (TM) recorder.Results Average clinic BP (+/- SD) was 167 +/- 12/ 90 +/- 8 mmHg. Average daytime ambulatory BP was 157 +/- 15*/89 +/- 10* mmHg and night ambulatory BP was 137 +/- 16(+)/74 +/- 10(+) mmHg (different from clinic BP: *P <0.01;from daytime ambulatory BP: P+ <0.001). Twenty-one to 45% of all patients had higher daytime systolic or diastolic ambulatory BP than clinic readings, with smoking, previous treatment for hypertension and lower clinic BP being the main predictors of this 'reverse white-coat effect.Conclusions Although mean daytime ambulatory blood pressures were lower than clinic readings in this large cohort of untreated older hypertensives, a substantial proportion showed the reverse of the so-called 'white-coat effect'. These findings identify the important role for ABPM in the elderly, not only for avoiding overtreatment in those with typical 'white-coat hypertension' but also for ensuring adequate treatment is given to those with the reverse of this phenomenon. J Hypertens 20:639-644 (C) 2002 Lippincott Williams Wilkins.