This report uses published information on the tenures of the 862 U.S. medical school deans who served from 1940-41 through 1990-91 to ascertain whether the turnover of deans has increased historically. The data confirm the wide-spread impression that there has been increasing instability of medical school leadership in recent decades. The proportion of deans who survive to a specific tenure in office has diminished, and the proportion of schools with new deans has increased, although with marked yearly fluctuations. Furthermore, the frequency of deans' turnover has not been evenly distributed among medical schools. Some schools have had large numbers of short-tenured deans, while other schools have had only a few deans over the five decades studied. The authors speculate that the rise in the turnover of deans may be related to the criteria used for their selection and/or because American academic medical centers have grown in size and are increasing in organizational complexity. The authors urge that future research that explores the causes of recent deans' turnover should incorporate modern management and statistical techniques and consider organizational variables as well as the personal and professional characteristics of deans.