OBJECTIVES: To perform an economic evaluation for the primary prevention of typeII diabetes based on the intermediate report of JDPP. METHODS: At first, SF36(V.1.20) and EQ-5D with Japanese version were applied, surveying over 205 participants, to assess whether or not the stepped care in JDPP may change the QOL of the patients with the relevant symptoms of silence. The second, a decision-analytic model was used to combine transition probabilities with clinical stages of diabetes, resource use and cost data in the framework of cost-effectiveness analysis within three years since 1998. The model employed a societal perspective to estimate the expected costs for each group of the stepped vs. ordinary cares which included the direct costs for intervention and treatment, and also production loss of patients as indirect costs. RESULTS: A total of 185 questionnaires on SF-36 and EQ-5D were returned (88 stepped, 97 ordinary). There was no statistically significant difference between the scores of two groups. The expected costs a patient in the decision model were estimated as US$4072 (US$=JPY110) for the stepped care, and US$2695 for the ordinary care with the discount of 3% a year in three years. The incremental cost-effectiveness ratio was US$17,636 in terms of cost per patient prevented from becoming Type-2 diabetes. CONCLUSIONS: The analysis on the JDPP intermediate report suggested that the stepped care resulted in increased costs for prevention comparing to the ordinary care in three years, maintaining the same level of QOLs in both groups.