The essential differences between the common, irregular medical check-up, as practiced in the follow-up clinics in England and Scandinavia, and the type practiced in the systematic follow-up clinic in the "Marburg experiment" are presented with examples. The necessity of the latter for clinical knowledge, for the attending surgeon, for special patients, and last but not least, for clinical research, is stressed. Based on examples from the Marburg systematic follow-up clinic for duodenal ulcer studies, its practicability is discussed.