It is often assumed that aging is a uniform process throughout adulthood because of the approximately linear increase of logarithmic mortality. We explored this assumption by analyzing cause-specific mortality increases in France (1979-1994). Rising rapidly at ages 30-54 years ("middle age") are death rates from malignant neoplasms at various sites, acute myocardial infarction, hypertensive disease, and liver cirrhosis. Steeply increasing at 65-89 years ("old age") are death rates from certain infectious diseases, particularly of the respiratory system; certain types of accidents; nonalcoholic mental disorders (probably due mainly to Alzheimer's disease and senile dementia); heart failure; cerebrovascular disease; and some "vague" categories. The processes at work may be fundamentally different in these two life history stages, such that the mortality rise in middle age reflects specific chronic diseases that develop prematurely in some high-risk individuals, whereas the mortality increase in old age is dominated by senescent processes that eventually raise the vulnerability of almost all individuals to multiple pathologies.