Hospital information systems are often huge and heterogeneous systems. To support physicians with their daily clinical work, application systems are developed which are dedicated to particular medical fields or tasks and which have to be integrated into the hospital information system. The integration process is quite complicated, because it makes the information system's infrastructure even more heterogeneous. We developed an application system for documentation and therapy planning in pediatric oncology (DOSPO) and we started to introduce it into the Department of Pediatric Oncology of Heidelberg University Hospital. The fact that DOSPO is developed as a universal system for nationwide use made the integration process more difficult. In any case, the introduction of specialized application systems has to be planned systematically in advance, regarding the prevailing information system's infrastructure, available resources and established processes. To simplify the integration process comprehensive electronic patient records for the future should be designed in a way that they can be enhanced easily by new clinical functions.