The prevalence of significant hypertension in childhood may be higher than expected. We have measured sitting blood pressure in 168 11-yr-old children attending a pre-high school in the Neapolitan area on four occasions over a 3-month period. We have also measured blood pressure and heart rate during a mental arithmetic test and during isometric exercise. Prevalence of significant hypertension (diastolic blood pressure 82-90 mmHg) was 13% at the first visit and decreased to 6.5% at the last visit; prevalence of severe hypertension (diastolic blood pressure greater than or equal to 90 mmHg) decreased from 5.4% to 1.2% from the 1st to the 4th visit. No sex-related difference was observed. A more frequently positive family history of hypertension (50% vs 30% in the fifth and first quintile of blood pressure, respectively) and higher body weight (body mass index = 21.4 vs 19.3 kg/m2) were found in children in the 5th quintile of blood pressure distribution. Blood pressure increased during mental arithmetic by 10/13% of the resting values in the first quintile and by 6/11% in the last one during mental test; during handgrip the increase was of 20/37% and 24/46%, respectively: differences between quintiles did not reach statistical significance. These data show that about 8% of a Neapolitan sample of school population have high blood pressure levels, while no difference in vascular reactivity to sympathetic stimulation was detected in children with higher blood pressure.