Background: A number of previous studies have suggested that the Japanese have few opportunities to participate in medical decision-making, as a result both of entrenched physician paternalism and national characteristics of dependency and passivity. The hypothesis that Japanese patients would wish to participate in treatment decision-making if adequate information were provided, and the decision to be made was clearly identified, was tested by interview survey. Methods: The subjects were diabetic patients at a single outpatient clinic in Kyoto. One of three case study vignettes (pneumonia, gangreneorcancer) was randomly assigned to each subject and, employing face-to-face interviews, the subjects were asked what their wishes would be as patients, for treatment information, participation in decision-making and family involvement. Results: 134 patients participated in the study, representing a response rate of 90%. The overall proportions of respondents who preferred active, collaborative, and passive roles were 12%, 71%, and 17%, respectively. Respondents to the cancer vignette were less likely to prefer an active role and were more likely to prefer family involvement in decision-making compared to non-cancer vignette respondents. If a physician's recommendation conflicted with their own wishes, 60% of the respondents for each vignette answered that they would choose to respect the physician's opinion, while few respondents would give the family's preference primary importance. Conclusions: Our study suggested that a majority of Japanese patients have positive attitudes towards participation in medical decision making if they are fully informed. Physicians will give greater patient satisfaction if they respond to the desire of patients for participation in decision-making.