Prospective registries performed in consecutive patients with frequent cardiac diseases and procedures are able to provide important information about therapies, complications, clinical outcome, and temporal trends in cardiology. Since 1992 all consecutive percutaneous coronary interventions (PCI) performed in over 80 hospitals of the ALKK (Arbeitsgemeinschaft Leitende Kardiologische Krankenhausärzte) have been centrally collected in a registry. This registry contains over 20% of all PCI procedures performed in Germany during the last 10 years. By restricting the documentation to a limited number of items, it was possible to establish this registry with a high degree of completeness of the data. A similar concept formed the basis for five consecutive registries in patients admitted with acute myocardial infarction. In these registries the participating investigators were asked to provide reasons for their treatment decisions, e. g., withholding guideline- adherent therapies. With this feature an increase in the use of guideline-adherent therapies could be observed, which was associated with an improvement in clinical outcome.