Ambulatory blood pressure monitoring is increasingly being used in clinical trials. Trials with ambulatory monitoring, just like clinical experiments based on conventional sphygmomanometry, need to be controlled properly, because ambulatory blood pressure measurements are not completely devoid of placebo-like effects. The trough: peak ratio might be a useful instrument for assessment of the durations of action of antihypertensive drugs. However, its error term and confidence interval need to be reported and its determination in clinical trials requires further standardization. Ambulatory compared with conventional blood pressure measurements are characterized by a higher reproducibility. This property makes a reduction in sample size possible in cross-over but not in parallel group trials, if instead of the conventional blood pressure the 24 h or daytime blood pressures are compared. Trials focusing on the full course of the blood pressure through the day need a larger sample size than do those just concerning the conventional blood pressure level.