OBJECTIVES: We define patients who have not consulted doctors to treat their diabetes, while they have learned their blood-sugar levels are high through health check-up, as untreated-diabetes patients. Our research objective is to calculate using real world data how much lower the medical cost would be if the untreated-diabetes 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 71 untreated-diabetes patients in a virtual (yet supposed-to-be typical according to the JMDC data) health insurance society with 10,000 members.  And 16% of them would visit doctors within 3 months, while remaining 84% would leave their conditions as they are for averagely 40 months knowing that their blood-sugar levels are high. It is necessary to advise untreated-diabetes patients to visit doctors for treatment.  Such advices should be able to start their diabetes treatment in early stages and prevent them from future complicating diseases. According to our calculation, the medical cost after its diagnosis would increase by 1.1% without aging factors by leaving their untreated-diabetes conditions for one month. CONCLUSIONS: If the virtual health insurance society had all the existing 71 untreated-diabetes patients visit doctors right now, their monthly medical cost would be 0.37 million yen lower against the amount they had to pay in the future (averagely in 20 months) if they continue to avoid visiting, which represents 37 yen a month per member, and all the patients with high blood-sugar levels visit doctors retrospectively, its monthly medical cost would have been 4.12 million yen lower now, which represents 412 yen a month per member.