Mortality patterns prevalent among American Indian youth have not been well documented. This investigation reports on mortality patterns among the Seneca Nation of Indians from January 1, 1955, through December 31, 1989. The study cohort consisted of 3,033 Seneca tribal members born during the study period. Deaths occurring among cohort members younger than age 25 were identified through a computer match against New York State vital record files. Sex-specific standardized mortality ratios were calculated on the basis of mortality patterns exhibited by the general population of New York State, exclusive of New York City. Males exhibited significantly elevated mortality for all causes combined, for deaths due to all accidents combined, for motor vehicle accidents, and for suicide. Females demonstrated significantly elevated mortality from all accidents combined, for motor vehicle accidents, and for all other types of accidents. Age-specific mortality patterns also varied both by sex and by calendar time. These findings are important to consider in the design of programs aimed at reducing premature mortality among American Indian populations from preventable causes of death.