Prospective studies require the storage of the participants' personal data for follow-up. This demands special precautions for the protection of these data. Additional to the informed consent of the participant which enables the researchers to collect the prospective study data, the participant's right of informal self-determination should be protected. In the EPIC (European Prospective Investigation into Cancer and Nutrition) Potsdam study, therefore, an internal trustee was established that collects follow-up information from participants, physicians, clinics, health offices, tumour and population registries with the aim to obtain valid information on vital status and diseases of the participants. The internal trustee is a spatial and organizational separated working group. The isolated electronic data processing network, the alarm system which controls the rooms of the working group, the access control for employees are special security arrangements. During follow-up the individual identification number of the study data base is replaced by a specific identification number solely for the follow-up. Further, unique identification numbers are allocated to every single action and every single document such as inquiry forms which are sent to physicians, health offices, or population registries, that allow anonymous mailing and processing of the collected information. On copies of important documents, the name is erased and replaced by a specific record number. The inquiry forms for the physicians, for instance, contain the name of the participant at the head which has to be cut off before returning the form to the internal trustee. The identification number is also printed as bar code on each document facilitating its processing with technical aids such as scanners. If checkbacks are necessary, only certain work group members are able to reconnect the document number with the name. Depending on the task, the computer programmes confine automatically the details presented to the assistant. All arrangements to protect personal data do not interfere with research interests; on the contrary they lead to an efficient work routine and guarantee follow-up according to international rules.