Increases in blood pressure during the period of emotional arousal attendant to impending exertion are well documented, yet the etiologic significance of these elevations is unknown. Research suggests that exaggerated cardiovascular responses to psychological stress may be importantly related to hypertension. We examined blood pressure reactivity in anticipation of an exercise stress test in relation to future hypertension in the Kuopio Ischemic Heart Disease Risk Factor Study, a population-based study of middle-aged men from Eastern Finland. Subjects were 508 unmedicated men with resting blood pressure less than 165/95 mm Hg who completed a bicycle ergometer stress test at baseline and whose hypertensive status was assessed at 4 years of follow-up. Systolic and diastolic reactivity were calculated as the difference between blood pressure measured after seated rest on the bicycle ergometer before initiation of exercise and mean seated resting blood pressure measured 1 week earlier. Logistic regression models adjusted for age and resting blood pressure revealed a graded association between quartiles of reactivity and risk of subsequent hypertension (greater or equal to 165/95 mm Hg), with men showing systolic responses greater than or equal to 30 mm Hg or diastolic responses greater than 15 mm Hg at nearly four times the risk of becoming hypertensive (odds ratios, 3.80 [95% confidence interval, 1.90 to 7.63] and 3.65 [95% confidence interval, 1.86 to 7.17], respectively) relative to the least-reactive groups (systolic response, < 10 mm Hg; diastolic response, < 5 mm Hg). Adjustments for traditional risk factors for hypertension did not alter these associations. Results demonstrate the clinical significance of the pressor response in anticipation of exercise and support the hypothesis that cardiovascular reactivity to psychological challenge plays a role in the etiology of hypertension. (Hypertension. 1996;27:1059-1064.)