This study examines the predictive accuracy of four different methods of administration of a questionnaire designed to predict cancer and coronary heart disease (CHD) in healthy probands. The method of administration uses the establishment of trust and the explanation of questions as variables in all four possible combinations, i.e. trust and explanation, trust only, explanation only, and neither, the prediction being that the combination of trust and explanation would produce the most accurate prediction, the treatment using neither the worst prediction, with methods using either trust alone or explanation alone intermediate. The criterion was the successful prediction of cancer and CHD. A total population of 3563 men and women was used, and followed up over 15 years, death certificates being used to establish cause of death. As predicted, the combination of trust and explanation did best, use of neither worst. Explanation seemed more important than trust, and the combination seemed to have a synergistic effect. It is apparent that method of administration had an important effect on the outcome of the experiment.