Primary hypertension is associated with disturbed activity of the sympathetic nervous system and altered blood pressure rhythmicity. We analyzed changes in cardiovascular rhythmicity and its relation with target organ damage during 12 months of antihypertensive treatment in 50 boys with hypertension (median, 15.0 years). The following parameters were obtained before and after 12 months of antihypertensive treatment: 24-hour ambulatory blood pressure, left ventricular mass, carotid intima–media thickness, and MRI for visceral and subcutaneous adipose tissue. Amplitudes and acrophases of mean arterial pressure and heart rate rhythms were obtained for 24-, 12-, and 8-hour periods. After 1 year of treatment, 68% of patients were normotensive, and left ventricular mass and carotid intima–media thickness decreased in 60% and 62% of patients, respectively. Blood pressure and heart rate rhythmicity patterns did not change. Changes in blood pressure amplitude correlated with the decrease of waist circumference ( P =0.035). Moreover, the decrease of visceral fat correlated with the decrease of 24-hour mean arterial pressure and heart rate acrophases (both P <0.05). There were no differences in changes of blood pressure and heart rate rhythms between patients who achieved or did not achieve normotension and regression of left ventricular mass and carotid intima–media thickness. It was concluded that abnormal cardiovascular rhythmicity persists in children with primary hypertension despite effective antihypertensive treatment, which suggests that it may be the primary abnormality. The correlation between changes in cardiovascular rhythmicity and visceral obesity may indicate that the visceral fat plays an important role in the sympathetic activity of adolescents with hypertension.