Previous studies have reported an increased risk of developing sustained hypertension (SH) in borderline or mildly hypertensive subjects showing an exaggerated response of blood pressure (BP) to mental stress. The aim of this study was to assess if the response of BP to mental stress tasks is an independent predictor of SH. A total of 89 patients with grade 1 hypertension, aged 18-64 years, 62% males, were included. The mean of follow-up was 5.3 years (s.d. 2.1 years). SH was defined as the development of grades 2-3 hypertension (Systolic BP>or=160 mmHg or diastolic BP>or=100 mmHg) or to be in antihypertensive treatment after follow-up. Two mental stress tasks: mental arithmetic stress task and a stressful interview (SI) were applied at entry. The subjects were classified as hyper-reactors when BP increase was greater than 35 mmHg for systolic BP or greater than 21 mmHg for diastolic BP, according to the results obtained previously in a normotensive control group. In the univariate analysis, the factors associated with the development of SH were age (P=0.0007), office diastolic BP (P=0.014) and hyper-reactivity of BP during a stressful interview (P=0.003). In the Cox regression model, after adjusting for gender, age, and office BP, the hyper-reactivity of BP during SI was an independent predictor of development of SH. In conclusion, the response of BP to mental stress tasks is useful in predicting SH in young and middle-aged subjects with grade 1 hypertension.