The effects on mortality of cognitive impairment and 3-year declines in cognitive function were examined among community-dwelling adults aged 68 years or more. Data were taken from a population-based cohort study that enrolled noninstitutionalized elderly residents of New Haven, Connecticut, and followed them by conducting in-home interviews in 1982, 1985, 1988, and 1994. The cognitive function of 1,997 respondents was assessed by using the 30-point Mini-Mental State Examination in 1985; 1,372 respondents (86% of those alive) were retested in 1988. Responses were classified as high normal (28-30), low normal (24-27), mild impairment (18-23), or severe impairment (0-17); cognitive decline was defined as a transition to a lower category. After control for multiple potential confounders, both severe and mild cognitive impairment were strongly predictive of subsequent mortality among respondents aged less than 80 years. Upon closer examination, the elevated mortality risk was observed primarily among respondents whose cognitive decline was recent rather than among those whose cognitive performance was compromised but stable. Among respondents aged 80 years or more, declines to severe cognitive impairment were predictive of mortality, but it was not clear whether the decline per se signaled an unfavorable prognosis not accounted for by the resulting impairment level. Cognitive declines, especially those in the young elderly, have a marked adverse impact on survival.