We measured physical fitness, assessed by maximal treadmill testing in 4,820 men and 1,219 women aged 20 to 65 years. Participants had no history of cardiovascular disease and were normotensive at baseline. We followed up these persons for one to 12 years (median, four years) for the development of hypertension. Multiple logistic risk analysis was used to estimate the independent contribution of physical fitness to risk of becoming hypertensive. After adjustment for sex, age, follow-up interval, baseline blood pressure, and baseline body-mass index, persons with low levels of physical fitness (72% of the group) had a relative risk of 1.52 for the development of hypertension when compared with highly fit persons. Risk of hypertension developing also increased substantially with increased baseline blood pressure.