Background Increasingly, prophylactic mastectomy has been evaluated as a treatment of breast cancer. Hereditary breast cancer now accounts for approximately 5%–10% of all cases of breast cancer, meaning that the widespread implementation of prophylactic mastectomy may significantly reduce the occurrence of breast cancer. However, prophylactic mastectomy is rarely performed in Korea. Therefore, in this study, we assessed Koreans' attitudes toward and awareness of preventive mastectomy. Methods This was a prospective study of a cohort of patients attending outpatient clinics and their relatives. Data were collected using self-administered questionnaires assessing sex, age, educational level, knowledge of breast cancer, understanding of prophylactic mastectomy, attitudes toward prophylactic mastectomy, and reasons for choosing prophylactic mastectomy. Results Sixty-five patients were included. Most patients (36.9%) were between 40 and 49 years of age and 58.4% were college graduates. Only six respondents (9%) understood prophylactic mastectomy, and 17 respondents (27%) stated that they would agree to undergo prophylactic mastectomy if necessary. Reasons given for refusing prophylactic mastectomy included aesthetic concerns (38%), the perception that it would not cure the disease (26%), possible surgical complications (24%), and financial cost (6%). Conclusions In this study, most of the respondents showed a poor knowledge of prophylactic mastectomy. Ultimately, it will be necessary to establish medical guidelines for patients with a high risk of breast cancer, with the objective of providing accurate information and proper treatment at hospitals.