Errors can be made when blood pressure is measured only by a single clinic measurement. Such assessments bear only a limited relationship to mean 24-h blood pressure and do not provide complete information on potentially important features of the circadian pattern of blood pressure. Measurement of clinic or "casual" cuff blood pressure by the doctor can produce a large, variable, and unpredictable overestimation caused by the pressor response associated with anxiety in the patient. Single cuff measurements rarely reflect the 24-h mean and classifications of severity based on cuff measurements are therefore subject to a large degree of overlap. Estimation of the response to antihypertensive treatment by casual cuff measurements is therefore subject to the same criticisms. Although superiority of measurement of 24-h mean blood pressure has not been demonstrated in any prospective controlled study, several cross-sectional investigations have shown that complications of hypertension relate more closely to the 24-h mean than to casual measurements. The degree of variability of blood pressure during a 24-h period (which can only be measured by 24-h monitoring) bears a relation to target-organ damage that is independent of mean blood pressure value. Nighttime blood pressure mean has been shown to correlate with target organ damage approximately as closely as daytime blood pressure mean, implying that antihypertensive drugs must act during the night as well as the day if they are to influence the complications of hypertension.