OBJECTIVES: We define patients who have not consulted doctors to treat their hypertensions, while they have learned their blood pressure levels are high through health check-up, as untreated-hypertension patients. Our research objective is to calculate using real world data how much lower the medical cost would be if the untreated-hypertension patients visit doctors in response to suggestions to do, which represents the cost reduction of cost-effectiveness analysis. METHODS: We used the data of Japan Medical Data Center (JMDC), which provides health insurance claims data with linked health check-up data of 1.7 million members from health insurance societies in Japan. RESULTS: It is estimated there are around 705 untreated-hypertension patients in a virtual (yet supposed-to-be typical according to the JMDC data) health insurance society with 10,000 members. They would leave their conditions as they are for an average of 6 years knowing that their blood-pressure levels are high. It is necessary to advise untreated-hypertension patients to visit doctors for treatment.  Such advices should be able to start their hypertension treatment in early stages and prevent them from future complicating diseases. According to our calculation, the medical cost after taking antihypertensive would increase by 11% without aging factors by leaving their untreated-hypertension conditions for one year. CONCLUSIONS: If the virtual health insurance society  had all the existing patients with mid-level (160/100 or higher) high blood pressures visit doctors right now, their monthly medical cost would be 0.71 million yen lower against the amount they had to pay in the future (averagely in 3 years) if they continue to avoid visiting, which represents 71 yen a month per member, and had all the existing patients with mid-level high blood pressures visit doctors retrospectively, its monthly medical cost would have been 0.52 million yen lower now. This amount represents 52 yen a month per member.