Background and Objectives: Even with appropriate donor deferrals and advanced screening tests, the risk of disease transmission through blood transfusion cannot be completely disregarded. Efficient monitoring of possible disease transmission between blood donors and recipients should be an important component of a comprehensive haemovigilance system. Materials and Methods: We assembled the Scandinavian Donations and Transfusions (SCANDAT) database, with data on virtually all blood donors and recipients who have been registered at least once in any of the computerized local blood bank databases in Sweden and Denmark since the start of computerized registration in 1966. The records of these individuals, with their entire computerized donation and/or transfusion histories and all donor-component-recipient connections, were linked to nationwide population and health registers to attain essentially complete follow-up for up to 36 years regarding reproduction, hospital morbidity, cancer, and death. Results: After data cleaning, the database contained 1 134 290 blood donors who contributed 15 091 280 records of donations and 1 311 079 recipients who received 11 693 844 transfusions. The data quality in the existing data sources was satisfactory. From the data obtained from local blood banks, 4·6%, 1·6%, and 6·4% of the person, donation, and transfusion records, respectively, had to be discarded after review of the legitimacy of recorded values, and comparisons with independent, external databases. Conclusion: It is possible to use existing computerized data, collected in routine health care, in haemovigilance systems for monitoring long-term outcome and disease concordance in blood donors and transfusion recipients.