In Japanese national health insurance system, there is some limit for monthly co-payment amount. In some diseases, AIDS, chronic renal failure with dialysis and haemophilia, co-payment limits are additionally reduced. Patient groups for myeloma, cervical cancer, hepatitis, chronic myelogenous leukemia and rheumatoid arthritis argued that co-payment reduction should be implemented to their diseases. OBJECTIVES: To conduct conjoint analysis to determine how people value various factors of diseases in decision-making process for co-payment reduction. METHODS: We determined six factors, i.e., influence for life years/QOL (low/high) , availability for medicines (yes/no) , number of patients (5,000/50,000 / 500,000), disease duration (short/long) and out-of-pocket expense per 1 month (JPY10,000/JPY100,000 , USD1=JPY90) and developed questionnaire. Ninety-six patterns are reduced to 26, via orthogonal methods. A total of 1163 participants filled questionnaire via web survey system. We adopted panel-logit model to estimate odds ratios for each factors. RESULTS: All 6 factors significantly influenced peoples' decision-making process. Coefficient for each factors were as follows; out-of-pocket expense: 2.61 > influence for QOL: 0.976 > disease duration: 0.884 > influence for life years: 0.831> number of patients: 0.105 > availability for medicines: -0.18, respectively. Based on this estimation, when we consider characteristics of anti-rheumatoid biologics, more than 95% people think that out-of-pocket rate should be reduced. CONCLUSIONS: Various factors contribute to people's attitude for co-payment reduction. This quantitative result would be helpful for decision-making process in national health insurance system.