Fitness and physical activity are each inversely associated with the development of hypertension. We tested whether fitness and physical activity were independently associated with the 20-year incidence of hypertension in 4618 men and women. Hypertension was determined in participants who had systolic blood pressure >or=140 mm Hg or diastolic blood pressure >or=90 mm Hg or who reported antihypertensive medication use. Fitness was estimated based on the duration of a symptom-limited graded exercise treadmill test, and physical activity was self-reported. The incidence rate of hypertension was 13.8 per 1000 person-years (n=1022). Both baseline fitness (hazard ratio: 0.63 [95% CI: 0.56 to 0.70 per SD]; 2.9 minutes) and physical activity (hazard ratio: 0.86 [95% CI: 0.79 to 0.84 per SD]; 297 exercise units) were inversely associated with incident hypertension when included jointly in a model that also adjusted for age, sex, race, baseline smoking status, systolic blood pressure, alcohol intake, high-density lipoprotein cholesterol, dietary fiber, dietary sodium, fasting glucose, and body mass index. The magnitude of association between physical activity and hypertension was strongest among participants in the high fitness (hazard ratio: 0.80 [95% CI: 0.68 to 0.94]) category, whereas the magnitude of association between fitness and hypertension was similar across tertiles of physical activity. The estimated proportion of hypertension cases that could be prevented if participants moved to a higher fitness category (ie, preventive fraction) was 34% and varied by race and sex group. Fitness and physical activity are each associated with incident hypertension, and low fitness may account for a substantial proportion of hypertension incidence.