Abstract Background: Ambulatory blood pressure monitoring (ABPM) has been found to be of significant importance in clinical practice because numerous blood pressure (BP) measurements may be made throughout the 24-hour period. Objective: To assess the clinical utility of ABPM in children with secondary hypertension. Methods: We studied 37 patients (21 boys and 16 girls), with a mean age of 16.4 ± 4.1 years, after kidney transplantation and 38 patients (27 boys and 11 girls), with a mean age of 10.2 ± 2.1 years, after surgical correction of aortic coarctation. Data, expressed as mean ± SD, were analyzed after dividing the patients into 4 groups. Group A consisted of 25 patients receiving antihypertensive therapy; group B included 12 patients not receiving antihypertensive therapy. Group C included 25 patients operated on before 3 years of age (8 ± 7 months of age); group D included 13 patients operated on after 3 years of age (74 ± 29 months of age). Results: In groups A and B, casual BP measurement showed that 16 of 37 patients (43%) were hypertensive; 24-hour ABPM detected a larger number of patients who were hypertensive (23 of 37, 62%); there were 18 in group A and 5 in group B. In groups C and D, casual BP measurement identified 6 of 38 (15%) patients as hypertensive, whereas 24-hour ABPM again identified a higher number (13 of 38, 34%). Conclusions: Our findings confirm that 24-hour ABPM is more sensitive than casual BP in detecting abnormal BP in patients at high risk for secondary hypertension.