Three recent observations when considered together indicate means whereby the health of Western countries could be improved and certain diseases still rare in developing countries avoided. The first is the recognition that many of the commonest chronic disorders in more economically developed countries are characteristic of modern Western lifestyles; the second is that these disorders can be considered to be manifestations of maladaptation to a new environment; and the third is the knowledge now available of the types of environment to which man has, through evolution, adapted.