Do the conditioning and training of professional athletes determine their length of life or cause of death? We have addressed this question through an analysis of the rate and causes of death of American professional baseball players. The cohort comprised all baseball players who played their first games for a professional major-league baseball team in the United States between 1911 and 1915 and who survived until 1925. Of 1030 players listed by one or more baseball publications as having made their professional debut between 1911 and 1915, 6 were excluded from the cohort because officials at the Major League Baseball. © 1988, Massachusetts Medical Society. All rights reserved.