This study employs vital statistics data from Sweden, England, Wales, France, and Spain to examine male:female mortality differentials from 1750 through 2000 and their inter-relationship with epidemiological transitions. Across all ages and all time periods, the largest relative mortality disadvantages are to young adult men. When crisis mortality from the two world wars is removed, the authors show that the mortality in this young male age group is about two to three times the level of female mortality cross-nationally. In addition, they show that the timing of this stabilization in male mortality disadvantages occurs during the last half of the twentieth century, when their measure of epidemiological change also stabilizes at a new low level. The findings are consistent with an interdisciplinary theoretical model that links social, technological and epidemiological changes that occurred through the first half of the 20th century with the unmasking of mortality disadvantages among young adult men.