Ambulatory blood pressure monitoring is invaluable in the assessment of the efficacy of antihypertensive drug treatment because it allows the reduction of blood pressure to be confirmed within the context of daily life. It also allows the determination of whether treatment produces an even blood pressure reduction over the 24-hour period or whether it is characterized by an uneven profile, for example, a precipitous fall early after administration of the drug and a later return towards elevated blood pressure values. A lack of information on prognostic and normal values does not permit quantification of the optimal blood pressure reduction to be achieved throughout the monitoring period. It is agreed, however, that the decrease should take into consideration both the elevated daytime and the reduced night-time blood pressures, and that peak blood pressure values associated with physical and emotional activities as well as the overall variability of blood pressure should also be reduced. Finally, ambulatory blood pressure monitoring is virtually devoid of the placebo effect, and the 24-hour average blood pressure data derived by this method are more reproducible than isolated blood pressure values taken by sphygmomanometry. This offers distinct advantages for studies designed to evaluate the antihypertensive effect of new drugs.