To identify the risk factors of borderline hypertension in the development of established hypertension, the relationship between the changes in blood pressure over 10 years and the clinical features in the initial year was investigated in a Japanese urban population. The occurrence of cardiovascular complications in cases with borderline hypertension were also studied. The prevalence of borderline hypertension in the initial year was 18.2%. On the basis of the blood pressure changes over 10 years, these subjects could be classified into the following 5 groups: (I) those with developing established hypertension, 19.0%; (II) those with fluctuating between hypertension and borderline hypertension, 27.4%; (III) those with remaining with borderline hypertension, 19.6%; (IV) those with fluctuating between borderline hypertension and normotension, 22.9%; (V) those with improving to normotension, 11.2%. The later development of established hypertension was found in only 2.4% of normotensives in the initial year. Obesity and glucose intolerance were more frequently observed in group I (38.2% and 29.4%, respectively) than in the other groups. Diastolic blood pressures in the initial year were significantly higher in groups I and II than in III, IV and V. As for the occurrence of cardiovascular complications during the observation period, electrocardiographic abnormalities and hypertensive and/or arteriosclerotic retinopathy were not infrequently observed even in borderline hypertensives. Therefore, in subjects with borderline hypertension, continuous medical management, including correction of obesity as well as glucose intolerance, is necessary to prevent the development of established hypertension and subsequent cardiovascular complications.