Changes of diet and other lifestyle factors are often highly recommended, but evidence for the effectiveness of behavioral changes in adult lives is limited. In the European Prospective Investigation into Cancer and Nutrition (EPIC)-Potsdam study, we investigated the reasons for and the direction of dietary changes as well as factors that are associated with the self-report of a change in diet. About 47% of the 27,548 cohort members reported a dietary change within the first 2 y of the follow-up. Detailed analysis of the type of change showed that public health recommendations are clearly understood. More vegetables and fruit and less fat are the most reported changes. Investigations into the reasons for dietary changes showed that in most instances and in ever shorter intervals, dietary recommendations resulting from progress in science are reinforced or partly modified. The health-conscious individuals in a population try to put these health messages into practice by modifying their behavior; these are the individuals who are particularly attracted to participate in scientifically oriented prospective studies and therefore are likely to be recruited by and retained in these studies. Behavioral changes over time may indicate dietary changes that are related to health and weight problems. The consideration of dietary changes during prospective data collection, therefore, is an important issue in the design and analysis of longitudinal studies-both cohort and intervention studies. Cohort studies in which dietary changes are adequately measured can contribute substantially to the evidence for health benefits resulting from changes in diet.