Increasingly, medical research is relying on data from healthcare provision. The systematic analysis of this data is a promising method that is particularly significant for quality assurance in the healthcare sector. However, this secondary use of treatment data raises numerous legal questions. Social and health data are among the most intimate information about a person and therefore require particularly high protection. The opportunities and risks must be carefully weighed against each other in light of the right to informational self-determination. This legal opinion provides, for the first time, a detailed overview of the specific legal frameworks for the secondary use of medical data in the individual federal states, taking into account the different responsibilities and regulations in each case.