This issue of Hypertension includes an article by Bur et al1 that focuses on the comparison between clinic and ambulatory blood pressure (ABP) values in patients with moderate to severe hypertension. The primary goal of the Bur study was to obtain a classification of hypertensive patients, based on the ABP values corresponding to the clinic blood pressure (BP) values that have been used to stage hypertension by the World Health Organization–International Society of Hypertension (WHO-ISH) and the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure (JNC) VI guidelines.2,3 An additional goal was to evaluate whether this ABP-based classification has prognostic value, as shown for the prognostic value of the clinic BP staging. Both clinic BP and ABP were measured in 736 hypertensive patients (557 of whom were under treatment) at the time of their first admission to the local Hypertension Unit. All patients then entered a follow-up period with an average duration of 52 months (range, 6 to 96 months), during which only clinic BP was obtained. During the observation time, 82 patients had nonfatal cardiovascular events and 9 patients died of cardiovascular causes. The article adds interesting information to the existing database on the clinical value of ABP. In particular, it contributes to the available knowledge on the prognostic importance of ABP as well as on its relation to clinic BP in the context of treating patients in a hypertension center.4 Stratifying patients into different risk categories on the basis of ABP values requires studies that (1) establish in populations or in large groups of hypertensive patients the relation of cardiovascular morbidity and mortality with the different 24-hour ABP values selected5,6 and (2) evaluate how prognosis of patients is modified when ABP is reduced by treatment, leading to a change in the ABPbased staging. This information is only partly available, however, because the association between the incidence of cardiovascular disease and ABP has been examined in only a few studies of suitable size.7 Furthermore, the few intervention studies addressing the prognostic value of treatment-induced changes in ABP have been undermined by problems such as a low number of BP measurements during ABP monitoring, use of surrogate end points rather than morbid or fatal events, uncontrolled experimental designs, small numbers of patients (and thus insufficient statistical power to back the study conclusions), and lack of ABP measurements during antihypertensive treatment.6 The available evidence clearly indicates that the upper limits of normal 24-hour average ABP are markedly 140/90 mm Hg. However, because of these problems, it does not provide any classification of hypertensive patients on the basis of ABP levels that might parallel the staging of hypertension, based on clinic BP, proposed by WHOISH and JNC VI guidelines.2,3 Bur et al1 have made an attempt to cope with at least some of the above problems. The results of their study confirm previous findings that the relation between clinic BP and ABP, although statistically significant, is by no means close. They also confirm the observations made in previous studies that ABP is significantly lower that clinic BP, especially in moderate and severe hypertension,4–7 because the discrepancy between clinic BP and ABP increases with increasing clinic BP values. Based on these findings, and consistent with previously published recommendations,2–7 the authors correctly emphasize that the classification of hypertension by WHO-ISH or JNC VI clinic BP criteria should not be directly transferred to ABP data. Finally, and most importantly, the results of Bur et al confirm and extend previous observations on the prognostic value of ABP by showing that staging hypertension by ABP values is indeed related to prognosis of hypertensive patients. It should be acknowledged that it is difficult to carry out controlled studies on the prognostic value of different ABP levels. This is because, as mentioned above, the study size needs to be large and the observation period to be prolonged in order to obtain a sufficiently high number of events that permit conclusions with high statistical power. In particular, if the purpose is not just to show the prognostic value of ABP (a rather likely finding) but whether ABP is prognostically superior to or adds to the prognostic value of clinic BP, the study size and duration need to be substantially increased. Finally, to assess whether cardiovascular protection depends more on treatment-induced reduction in ABP than in clinic BP (or whether knowledge of ABP reduction by treatment adds to the estimate of protection based on clinic BP reduction), additional requirements need to be fulfilled. These include (1) frequent collection of ABP data during treatment, (2) need to avoid excessive treatment nonhomogeneity, and The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Department of Clinical Medicine, Prevention, and Applied Biotechnologies, University of Milano-Bicocca; and II Cardiology Unit, S. Luca Hospital, IRCCS, Istituto Auxologico Italiano, Milano, Italy. Correspondence to Dr Gianfranco Parati, Cardiology II, S. Luca Hospital, Via Spagnoletto, 3 Milano–20149, Italy. E-mail email@example.com (Hypertension. 2002;40:792-794.) © 2001 American Heart Association, Inc.