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Background and Objectives: A major limitation of dietary trials is that double blind design is not feasible. These trials are therefore prone to biases. The Lyon diet heart study is a single-blind secondary prevention trial to test the hypothesis that a Mediterranean-type of diet may prevent recurrences after a ®rst myocardial infarction. A surprising 73% reduction of the risk of new major cardiac events was observed in the experimental group. For this reason, it is important to describe the methods used in the trial. We now report our techniques to randomize the patients, to change their diet and to control for possible bias, in particular any investigator or attending physician bias. Design: In this dietary trial, a speci®c design was used to recruit and randomize the patients without informing them and their physicians that they were participating in a comparative trial. The attending physician bias was evaluated by studying drug usage and the investigator bias by constructing a questionnaire from which speci®c scores were used to evaluate (1) how the patients appreciated their participation in the study and (2) whether this participation resulted in signi®cant changes in their way of living. Subjects: 605 survivors of a ®rst myocardial infarction were randomized into either a control or a Mediterranean group. Results: The two randomized groups were similar for all the variables of prognosis. Drug usage was not signi®cantly different between groups, suggesting that there was no major attending physician bias. Analyses of the appreciation scores and of the change score did not detect any signi®cant investigator bias. Conclusions: Although the study cannot be completely shielded from minor biases, the data presented here provide evidence that the dietary modi®cations per se were protective, not other (including psychosocial) changes resulting from the participation to the trial. Descriptors: dietary trial; control of biases; coronary heart disease; Mediterranean diet; attending physician.

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