Aim of this study was to assess blood pressure (BP) response to exercise in borderline hypertensive subjects and to evaluate its predictive value for subsequent established hypertension development: 74 male subjects (28 borderline subjects, 26 normotensive subjects and 20 subjects with established hypertension) underwent a maximal exercise testing in the sitting position with a bicycle ergometer at the beginning of the study and then after 1 and 2 years; besides casual BP was controlled every 3 months for 2 years. Systolic BP exceeding 220 and/or diastolic BP exceeding 105 mmHg at maximal exercise and/or diastolic BP exceeding 100 mmHg at th fifth min of recovery were considered as abnormal. On the basis of BP response to exercise we divided our study group in: normotensive subjects with a normal BP response (Group A: 88%); normotensive subjects with an abnormal BP response (Group B: 12%); borderline subjects with a normal BP response (Group C: 46%); borderline subjects with an abnormal BP response (Group D:56%). At the end of a 2-year follow-up established hypertension developed in 1 subject of Group B (33%), in 2 subjects of Group C (15%) and in 10 subjects of Group D (67%); 7 subjects of Group C returned to normotension (54%). The incidence of established hypertension is significantly higher in borderline subjects with an abnormal BP response to exercise. This finding is probably due to both functional and organic factors and stresses the predictive value of exercise testing in borderline hypertension.