Objective:The clinical significance of isolated systolic hypertension (ISH) in youth is controversial. One main confounding factor is the strong white-coat effect often observed in ISH patients. The aim of this study was to investigate the risk of hypertension needing pharmacological treatment in ISH identified with ambulatory 24-h blood pressure (24-h BP).Methods:We examined 1206, 18-45-year-old participants from the Hypertension and Ambulatory Recording VEnetia STudy. Based on 24-h BP, 269 participants were normotensive, 209 had ISH, 277 had isolated diastolic hypertension, and 451 had systolic-diastolic hypertension. The predictive role of ISH for incident hypertension was evaluated in Cox survival analyses, adjusting for risk factors and confounders.Results:ISH participants were more frequently young men active in sports, with lower heart rate and cholesterol. During a 6.9-year follow-up, 61.1% of participants developed hypertension. ISH participants had a nonsignificant increase in risk of hypertension compared with normotensive (reference group). In contrast, participants with diastolic hypertension (1.44; 1.13-1.85) or systolic-diastolic hypertension (2.04; 1.59-2.64) had a significant increase in risk. When the ISH participants were divided according to whether 24-h mean BP was normal (<97mmHg) or high, ISH patients with normal mean BP had no increase in risk (1.01; 0.73-1.40), whereas those with high mean BP had a significant increase in risk (1.70; 1.16-2.49).Conclusion:These data obtained with ambulatory BP monitoring show that in ISH people younger than 45 years, only mean BP is a predictor of future hypertension needing treatment, whereas the ISH status per se does not necessarily imply an increase in risk.