The authors examined the association between cardiorespiratory fitness and incident hypertension in women who were normotensive and free of cardiovascular disease at baseline in the Aerobics Center Longitudinal Study (Dallas, Texas), 1970-1998. A total of 4,884 women performed a maximal treadmill exercise test and completed a follow-up health survey. During an average follow-up of 5 years, 157 incident cases of self-reported, physician-diagnosed hypertension were identified from the health surveys. The cumulative incidence of hypertension was 3.2%. Compared with the rates of low-fit women, crude hypertension rates were 60% and 79% lower among women in the moderate and high fitness categories, respectively (p < 0.001). After adjustment for several potential confounders, the odds ratios for hypertension were 1.0, 0.61 (95% confidence interval (CI): 0.30, 1.21), and 0.35 (95% CI: 0.17, 0.73) in low-, moderately, and highly fit women, respectively (p(trend) < 0.01). Each 1-metabolic equivalent increment in treadmill performance was, on average, associated with a 19% (95% CI: 10, 27; p < 0.001) lower odds of incident hypertension. The pattern and strength of association between fitness and hypertension persisted in analyses stratified on body mass index, age, and the presence of prehypertension at baseline. An active lifestyle should be promoted for the primary prevention of hypertension in women.