The aim of the study was to compare the antihypertensive efficacy of once-daily lisinopril vs enalapril both during normal daily activity and sleep, in mild-to-moderate essential hypertension. After a 4-week wash-out period, 34 patients (17 M, 17 F) aged 22 to 67 years were randomized in a multicenter, open, parallel fashion: 17 received lisinopril (10-20 mg) and 17 enalapril (10-20 mg) for a 12-week period. Twenty-four hour ambulatory blood pressure monitoring (ABPM) was performed using an oscillometric non-invasive automated device at both the end of the 4-week drug-free baseline period and during the last week of treatment. With no differences in initial blood pressure (BP) between groups, both drugs significantly reduced office and ABPM values. Lisinopril tended to reduce BP in a greater extension than enalapril, but only the reduction of office systolic BP (SBP) (p = 0.0062), 24-h SBP load (P = 0.0182) and night time SBP load (P = 0.0316) reached statistical significance. We conclude that, in spite of a more prominent reduction of SBP by lisinopril, both drugs have a similar efficacy in reducing BP, assessed by both office and ABPM measurements.