The Swedish Personal Identity Number (PIN) is a unique identifier used in Swedish health care and national health registers. This paper explores the structure, assignment, and ethical implications of using the PIN in medical research. The PIN consists of a date of birth, a three-digit birth number, and a check digit, with the birth number being sex-specific. It is maintained by the National Tax Board for individuals residing in Sweden since 1947. The PIN is crucial for linking medical registers and achieving nearly 100% coverage of the Swedish health care system. However, issues such as incorrect PINs, re-use of PINs, and changes in PINs due to immigration or sex reassignment pose challenges. The paper discusses the ethical considerations of using the PIN in medical research, emphasizing the benefits of access to stored medical data for patient care and research. It argues that informed consent is not required for large-scale medical research, as it would hinder the statistical power of studies and potentially discriminate against high-risk groups. The authors conclude that the benefits of using the PIN in medical research outweigh the potential harms, provided that precautions are taken to protect patient privacy.The Swedish Personal Identity Number (PIN) is a unique identifier used in Swedish health care and national health registers. This paper explores the structure, assignment, and ethical implications of using the PIN in medical research. The PIN consists of a date of birth, a three-digit birth number, and a check digit, with the birth number being sex-specific. It is maintained by the National Tax Board for individuals residing in Sweden since 1947. The PIN is crucial for linking medical registers and achieving nearly 100% coverage of the Swedish health care system. However, issues such as incorrect PINs, re-use of PINs, and changes in PINs due to immigration or sex reassignment pose challenges. The paper discusses the ethical considerations of using the PIN in medical research, emphasizing the benefits of access to stored medical data for patient care and research. It argues that informed consent is not required for large-scale medical research, as it would hinder the statistical power of studies and potentially discriminate against high-risk groups. The authors conclude that the benefits of using the PIN in medical research outweigh the potential harms, provided that precautions are taken to protect patient privacy.