The use of ambulatory blood pressure monitoring in clinical studies offers some advantages in comparison to the clinic blood pressure measurement. In fact, this approach does not induce any alerting reaction and provides 24-h blood pressure values that are more reproducible and not affected by the placebo effect. This allows a better evaluation of blood pressure under antihypertensive treatment and an optimization of the number of patients to be studied in pharmacologic trials. In a recent double-blind, parallel-group study, ambulatory blood pressure monitoring was used to investigate the antihypertensive efficacy of a new angiotensin-converting enzyme inhibitor, trandolapril, in 62 mild and moderate hypertensive patients. After a washout period, patients received trandolapril, 2 mg o.d., or placebo for 6 weeks, followed by a second washout period. Clinic and 24-h blood pressures were assessed at the end of each period. In comparing the pre- and post-treatment period, trandolapril significantly reduced clinic and 24-h systolic and diastolic blood pressures. The fall was evident throughout the 24 h and was statistically significant also in the last 4 h of blood pressure monitoring. The placebo group did not show any significant blood pressure change. Thus, trandolapril, 2 mg once daily, is effective in reducing blood pressure. Its efficacy over 24 h is better documented by 24-h blood pressure monitoring than by isolated clinic blood pressure measurement.