BACKGROUND: Prehypertensive individuals are at increased risk for developing hypertension and cardiovascular disease compared to those with normal blood pressure (BP). Physically active, normotensive individuals are also at lower risk for developing hypertension than sedentary individuals. We assessed the relationship between fitness and 24-h ambulatory BP in prehypertensive men and women. METHODS: We assessed exercise capacity and 24-h BP in 407 men (age 51 +/- 11 years) and 243 women (age 54 +/-10 years) with resting systolic BP 120 to 139 mm Hg and diastolic BP of 80 to 89 mm Hg, defined as prehypertension. Fitness categories (low, moderate, and high) were established according to exercise time and age. RESULTS: Multiple regression analysis revealed that fitness status was inversely associated with ambulatory BP in both genders (P < .001). After adjusting for various confounders, individuals in the lowest fitness category had significantly higher 24-h, daytime, and night-time BP than those in the moderate and high fitness categories. For men, differences between low and moderate fitness categories were 6/4 mm Hg, 8/4 mm Hg, and 7/3 mm Hg for 24-h, daytime, and night-time BP, respectively (P < .05). For women, the differences were 8/5 mm Hg, 9/5 mm Hg, and 8/7 mm Hg for 24-h, daytime, and night-time BP, respectively. Similar differences were evident in both genders between low and high fitness category (P < .05). CONCLUSIONS: Moderate physical activity promotes lower BP during a 24-h period in prehypertensive men and women. The risk for developing hypertension is likely to be lowered if moderate intensity physical activity in this vulnerable population is encouraged.