Determining the biological limits to human longevity is more difficult than for most other species because humans are long-lived. Consequently, mortality data, such as from the U.S. vital statistics system, which have been available for a long time (relative to most epidemiological studies) and have large numbers of cases, including deaths reported to advanced ages, are important in studying human longevity-though care must be exercised in dealing with error in age reporting. Furthermore, it is unlikely that free-living humans can realize as much of their biological endowment for longevity as animals living in a highly controlled experimental environment. We examined changes, 1960 to 1990, in U.S. White male and female extinct cohort life tables and age at death distributions to (a) examine evidence for the effects of a biological life span limit in current U.S. mortality patterns and (b) produce lower bound estimates of that limit.