The conventional blood pressure (BP) measurement is essentially a 'snapshot' at a single time point in any given 24 h period. In the clinical management of the hypertensive patient, however, it is assumed that this snapshot is representative of the BP throughout 24 h and indicative of the consistency of the control produced by anti-hypertensive drug treatment. However, anti-hypertensive drugs vary in the duration and consistency of their anti-hypertensive effect and a single measurement of BP can only be reliably indicative if the drug effect is known to be consistently maintained throughout the 24 h. Following the deliberations of the Food and Drug Administration in the USA, the calculation of a trough:peak ratio has been proposed as an index of the consistency of the anti-hypertensive response and the suitability of an anti-hypertensive drug for its chosen dose and dose interval. In brief, to confirm that the magnitude of the BP is relatively consistent throughout 24 h it is recommended that the magnitude of the BP reduction at the end of the dose interval (at trough) should be at least 50% of the BP reduction measured at peak, namely a trough:peak of 50%. The trough:peak ratio provides a clinically relevant index for assessing whether an anti-hypertensive drug is likely to provide a consistent anti-hypertensive effect throughout the 24 h.