BACKGROUND: Weight reduction has been accompanied with a reduction in clinic blood pressure (BP) in children and adolescents; however, the effect on ambulatory BP (ABP) is uncertain. The objective was to investigate the impact of weight changes on ABP in obese children and adolescents. METHODS: Sixty-one severely obese patients aged 10-18 years underwent lifestyle intervention at the Children's Obesity Clinic. Patients were examined with ABP monitoring at baseline and after 1 year of treatment (follow-up). To account for growth, BP and BMI were standardized into z scores, whereas waist circumference was indexed by height [waist/height ratio (WHR)]. RESULTS: Patients experienced a reduction at follow-up in the degree of obesity [ΔBMI z score: -0.21, 95% confidence interval (CI) -0.32 to -0.10, P = 0.0003; and ΔWHR: -0.02, 95% CI -0.03 to -0.004, P = 0.009]. Δ24-h, Δdaytime and Δnight-time SBP and DBP in mmHg and changes in equivalent z scores were related to ΔBMI z scores and ΔWHR. These relationships were reproduced in multiple regression analyses adjusted for relevant confounders, for example, a reduction in one BMI z score corresponds to a reduction in 24-h SBP by 6.5 mmHg (P < 0.05). No relationship was found between changes in these anthropometric obesity measures and changes in clinic BP. CONCLUSION: Changes in obesity measures were closely related to changes in ABP, but not to changes in clinic BP, in severe obese children and adolescents after 1 year of lifestyle intervention. The findings emphasize the use of 24-h ABP measurements in children and adolescents.