Disease competition is a condition in which death rates are not what would be expected from the combination of etiological factors present in a region. Four types of disease competition are described: error; dominant occupational-lifestyle etiology leading to dominant diseases; dominant lifestyle with a variety of disease outcomes; and protective effect. Three clues that disease competition exists are discussed. In order to assess the importance of disease competition, an analysis was made of the geographical distribution of male white mortality from 23 causes in the 73 most populous countries in the United States. The results showed evidence only of the dominant lifestyle type.