1. The relationship between physical fitness (maximal oxygen uptake VO2max) and incidence of hypertension was investigated through a prospective study for a total of 16,525 human-years of observation. 2. This study involved 3305 Japanese males whose blood pressure (BP) was normal when they received their first physical examination before the age of 50. They were monitored from 1983 to 1988. The BP of 425 subjects was diagnosed as hypertension in the fifth year. 3. Fitness levels were divided into quintiles according to VO2max levels, and were compared with the changes of BP and relative risk of hypertension after adjustment for age, initial percentage of body fat (PFAT), initial BP, alcohol consumption, cigarette smoking status and familial history of hypertension. The increase in BP of subjects in the least fit group was higher than in any other group. Relative risk was calculated using a multiple logistic regression and was 1.9 x higher in the least fit group compared with the fittest group. 4. The subjects were classified into three groups: the improved VO2max group, the deteriorated VO2max group and the unchanged VO2max group. The increase in BP of the improved VO2max group was significantly lower than the other two groups after adjustment for changes in PFAT, age, initial PFAT, initial BP, fitness level, alcohol consumption, cigarette smoking status and familial history of hypertension. 5. It is concluded that low VO2max level is related to higher incidence of hypertension. An improved VO2max would therefore be able to prevent hypertension.