OBJECTIVE We aimed to evaluate the effects of blood pressure (BP) values obtained by 24-h ambulatory BP monitoring (ABPM) of hypertensive children and adolescents on left ventricular mass index (LVMI). METHODS Patients diagnosed with HT with BP measurements confirmed with ABPM and evaluated with echocardiography for LVMI were included. The patients were divided into two groups according to their BMI as obese and nonobese. SDSs of ABPM parameters were compared between the groups. RESULTS A total of 158 children with HT were included in the study. Ninety of these patients were obese. In obese and nonobese cases, mean SDS levels were similar in ABPM parameters, whereas LVMI was significantly higher in obese patients ( P = 0.049). There was a significant correlation between LVMI and 24-h SBP SDS, daytime SBP SDS, 24-h SBP load and daytime BP load. In obese cases, there was a statistically significant correlation between LVMI and 24-h SBP SDS, daytime SBP SDS, 24-h SBP load, daytime SBP load as well as nighttime SBP SDS and nighttime SBP load. When the whole group was evaluated, 24-h SBP SDS was the most effective parameter influencing LVMI ( P = 0.001). Similarly, the most effective ABPM parameter on LVMI in obese patients was 24-h SBP SDS ( P = 0.001). CONCLUSIONS A significantly higher rate of LVMI in obese patients suggests that obesity itself is an effective factor on LVMI. In addition, systolic hypertension is more effective on cardiac functions compared with DBP measurements and systolic-DBP dipping ratios.