Background and aimsThe Hypertension Optimal Treatment (HOT) study showed that when antihypertensive treatment reduces diastolic blood pressure well below 90 mmHg, there can be a further reduction of cardiovascular events, particularly myocardial infarction, with no evidence of a J-shaped curve at lower pressures. Office measurement, however, gives no information about blood pressure outside the office. This paper describes a HOT substudy in which patients underwent both office measurement and 24 h ambulatory blood pressure monitoring.MethodsThe mean age of the substudy population was 62±7 years. Substudy patients were treated for a median period of 2 years. All received the dihydropyridine calcium antagonist felodipine, while some also received an ACE-inhibitor, a beta-blocker or a diuretic. Average 24 h, day and night ambulatory blood pressure values were computed at baseline (n= 277) and during treatment (n= 347): 112 patients had been randomized to a target office diastolic blood pressure ⩽ 90 mmHg, 117 to⩽85 mmHg and 118 to⩽80 mmHg. Additional analyses included computation of: (1) trough-to-peak ratio and (2) the smoothness index (the ratio between the average of the 24 hourly blood pressure reductions after treatment and its standard deviation).ResultsTaking the subgroup as a whole, baseline 24 h average blood pressures (146±18/90±10 mmHg) were significantly and markedly lower than office blood pressures (170±14/105±3 mmHg,P< 0.01). Office, 24 h, day and night blood pressures were all significantly reduced by treatment, but there was a smaller fall in ambulatory, than in office pressures. The between group differences in office blood pressure were smaller than those observed in the overall HOT sample. Between-group differences in 24 h blood pressure were even smaller. Trough-to-peak ratios and smoothness indices were lowest in the highest blood pressure target group and highest in the lowest blood pressure target group. Office and ambulatory blood pressures were similar in the groups randomized to placebo (n= 170) or acetylsalicylic acid (n= 177).ConclusionIn conclusion, in the HOT study, treatment reduced not only office but also ambulatory blood pressure throughout the 24 h. The reduction was less marked for ambulatory than for office blood pressure.