All 995 persons with Down's syndrome who died in the United States during 1976 and whose death certificates listed Down's syndrome as the underlying or a contributing cause of death were identified. This allowed the underlying causes of death of 793 affected persons to be analysed and compared to deaths in the whole US population for that year. Mortality ratios provided evidence that the excess risk of leukemia mortality continues into adulthood and that deaths from other hematopoietic malignancies also occur excessively among Down's syndrome adults. Congenital anomalies of all kinds in infancy and congenital defects of the heart in infancy and later were also excessive. Respiratory tract infections and pneumonia showed persistently high ratios. Diabetes was raised only at ages 24 to 34 years. Ischemic heart disease, non-hematopoietic cancers, accidents, suicides and violence were under-represented among the causes of death. Methodological limitations of proportional mortality analysis are discussed.