The results of the German National Health Interview and Examination Survey will be of great importance for health policy and research. Therefore, internal quality management was supplemented by an external quality control which was carried out by a private company (Bernhard Schwertner Feld organisation, Augsburg). The interviewer training, sampling and response, field work and data management were included into the external quality control. For each of these four areas, measures of control and detailed check-lists were provided by the external quality control. The aim was to find sources of potential and real errors and to arrive at recommendations for internal quality management. The general concept of external quality control is briefly presented and experiences and results from the quality control of the field work are described in the present paper.