We investigated risk perception as it appears to breast cancer patients and how it develops. Wealth of anecdotal evidence as well as interview with both medical professionals and patients led us to hypothesize that patients' risk attitude would develop from incipience, wherein they are preoccupied by optimism toward complete treatment, to recurrence, wherein they become less optimistic and accept the realistic need to cohabitate with the disease. One-hundred breast cancer patients were recruited via voluntary patients organizations in Japan. The participants responded to either a 5-paged web questionnaire or a paper-pencil survey to judge the likelihood of certain risky treatment incidents happening to them, such as bad treatment result, side effects of chemotherapy, recurrence, and medical accident. The result showed that breast cancer patients did not differ as a whole in their optimism on medical risk perception between the two treatment stages. However, the result differed depending on patients' current treatment status: only in the regular treatment (defined as seeing a doctor once or twice a month) group patients were more optimistic in their incipient stage when compared with their recurrence stage, while there was no such difference in no treatment (defined as receiving a follow-up examination once a year) group. There was no difference in their optimism on medical risk perception among formally (medically) categorized cancer stages. Also, confirmatory factor analysis revealed that there were three distinctive factors to which the patients' optimism on risk perception was divided, namely “recurrence,” “aggressive treatment,” and “medical accident.” Hence, we uncovered a possible structure underneath “medical risk perception,” as well as successfully replicated the result of the past study. Implications and possible extension are discussed.