Hypertension and related cardiovascular diseases may lead to an increase in medical costs for patients. We attempted to clarify the relationship between hypertension and long-term medical costs by a cohort study utilizing existing data as well as baseline blood pressures and medical costs over a 10-year period. The participants included 4191 Japanese National Health Insurance beneficiaries aged 40-69 years, living in one area, who were not taking anti-hypertensive medication and did not have a history of major cardiovascular disease. They were classified into four categories according to their blood pressure. We evaluated the mean medical costs per month, cumulative hospitalization, and all-cause mortality for each blood pressure category. Hypertension-related medical costs attributable to hypertensive individuals, as compared to normotensive individuals, were estimated. There was a positively graded correlation between blood pressure and personal total medical costs, especially for men. The odds ratio for cumulative hospitalization and hazard ratio for all-cause mortality in severe hypertensive men were also higher than those in normotensive men. However, the hypertension-related medical costs for mild to moderate hypertensives were higher than those for severe hypertensives. The hypertension-related medical costs for all hypertensives accounted for 23.7% of the total medical costs for the Japanese population. In conclusion, high blood pressure was a useful predictor for excess medical costs; moreover, concomitant hypertension, regardless of the grade, increased the medical costs of Japanese National Health Insurance beneficiaries.