Demographic researchers have identified a crossover pattern between the mortality rates of the Caucasian-American and African-American oldest-old (80+) populations for over a century. Debate has centered on whether the crossover effect is due to age misreporting or the heterogeneity hypothesis or if it continues beyond age 99. This thesis addresses these issues by using new data from the SSA’s study of supercentenarians. The study identified 355 persons aged 110 or older whose ages could be verified, creating the first reliable American dataset for this population group. Analysis of the data has indicated that mortality rates at ages 110-115 were significantly lower for African-American supercentenarians than for their Caucasian-American counterparts, and that the African-American proportion of the population increased steadily with age. The results of this analysis show that the crossover phenomenon is multicausal and cannot be fully accounted for by age misreporting, suggesting a need to consider genetic and environmental impacts on racial variations in maximum human longevity.