OBJECTIVES: After bills to launch the Social Security and Tax Number System were enacted in 2013, health and political officials have considered the Healthcare Number System (the System). However, little is known about doctors' awareness and concerns about the System. This study aimed to measure how many doctors disagree with the System, examine the doctors' characteristics, and analyze the benefits and harms of the System that they identified. METHODS: A cross-sectional survey was conducted of doctors via the Internet. The participants were selected from a convenience sample of a panel of doctors based on stratified sampling including four groups: working at a hospital and <45 years; working at a hospital and ≥45 years; working at a clinic and <45 years; and working at a clinic and ≥45 years. The main outcome was how many doctors agreed or disagreed with the System. The prevalence was calculated for each group, and their characteristics were examined using multivariable logistic regression analysis. The responses to open-ended questions concerning the Systems' benefits and harms were analyzed using qualitative content analysis. RESULTS: There were 562 respondents (68%). By group, 16/143 (11%), 25/138 (18%), 31/132 (23%), and 43/149 (29%) doctors, respectively, thought that the System was unnecessary. The variables that correlated with the main outcome were age (per 5 years; odds ratio [95% confidence interval], 1.14 [1.01-1.29]) and type of medical facility (working at a clinic; 1.99 [1.30-3.08]). The doctors identified that unifying information could decrease administrative duties, facilitate inter-facility collaboration, and prevent inappropriate medical consultations. This could result in decreased healthcare costs and personalized healthcare. However, the doctors also identified that integrating information and dealing with big data could increase information leakage and information management, cause over-monitoring of doctors, and enable the inappropriate use of integrated information. This could result in deteriorating healthcare. Since some information should not be integrated, the System raises ethical considerations about privacy. CONCLUSION: Among the doctors surveyed here, 10-30% thought the System was unnecessary. These respondents tended to be older and work at a clinic. The System could decrease the cost of healthcare and enable personalized healthcare but could also increase information leakage and information management, cause over-monitoring of doctors, and enable the inappropriate use of integrated information. Prior to System introduction, we should facilitate consensus-building about protecting and utilizing personal information as well as consider the related ethical issues, and doctors' characteristics and concerns.