Background. The epidemiologic observation that physical activity reduces the risk for hypertension has only keen made for white men who self-reported hypertension. This study examined physical activity and clinically determined incident hypertension in black rand white men and women of the Atherosclerosis Risk in Communities (ARIC) Study. Methods. ARIC is a population-based prospective study with four U.S. clinic centers. The present analyses included 7,459 black and white adults 45-65 years of age. Hypertension (systolic/diastolic blood pressure greater than or equal to 140/90 mm Hg) was defined by blood pressure measured by a random-zero device or medication use. Physical activity was assessed with the Baecke questionnaire. Results. After adjustment for age, baseline blood pressure, ARIC center, education, body mass index, waist-hip ratio, parental history of hypertension, cigarette smoking, alcohol consumption, and diet, white men in the highest quartile of leisure activity (primarily cycling and walking) had a 34% lower odds of developing hypertension over 6 years compared to the least active (OR = 0.66, 95% CI = 0.41-0.94; P for quartile trend = 0.01). Baseline activity was not associated with incident hypertension in white women or blacks. Conclusions. Leisure-time physical activity reduces the odds of hypertension in middle-aged white men. Additional studies in women and blacks are needed. (C)1999 American Health Foundation and Academic Press.