Analysis of mortality data for the United States from 1900 through 1986 demonstrates near perfect Gompertzian mortality rate distributions for adult American men and women. In the U.S. between 1900 and 1986, annual age-adjusted mortality rate distributions were determined by a fixed common intersect point (for men, the mortality rate at age 87.41 years was 18,810/100,000; for women, the extrapolated mortality rate at age 108.69 years was 75,365/100,000) and a variable environmental factor. Despite living in the same environment, the environmental factor contributing to adult mortality is at present significantly less for women than men. However, analysis predicts that in an environment less conducive to human survival than has existed in the United States during this century, the environmental factor contributing to adult mortality was less for men than women. The study further implies that as the environment becomes more favorable for human survival, men will experience an effective lowering of their theoretical maximal life span toward a limit of 87.4 years. The calculated maximal life span for men has already decreased 7 years during this century. This negative effect of a more favorable environment also occurs in women, although the theoretical maximal life span in women is lowered only toward a limit of 108.7 years.