Patterns of mortality among members of the Seneca Nation of Indians between January 1, 1955, and December 31, 1984, were investigated. The study cohort consisted of all members of the Seneca Nation residing in New York State who were listed in the tribal rolls as of January 1, 1955 (n = 3,262). Deaths among cohort members were identified through a computer match against New York State vital records files. Sex-specific standardized mortality ratios (SMRs) were calculated on the basis of mortality patterns exhibited by the general population of New York State, exclusive of New York City. Seneca Nation males demonstrated an excess of deaths from all causes (SMR = 124), while all-cause mortality among Seneca Nation females did not differ from that expected (SMR = 106). Both males and females exhibited excess mortality from infectious diseases, diabetes mellitus, cirrhosis of the liver, and accidents and injuries. Excess mortality was also noted among males for deaths due to atherosclerosis and hernia/intestinal obstruction and among females for deaths due to pneumonia, chronic nephritis, and homicide. Both sexes exhibited a deficit of deaths due to malignant neoplasms and circulatory diseases. Findings from this study will be useful to those responsible for the planning and implementation of health care programs among the Seneca Nation of Indians and other Native American groups.