The association between educational attainment and decline in cognitive function over an interval of 1 year was examined for 14,883 subjects 18 years and older in the National Institute of Mental Health Epidemiologic Catchment Area Study. Cognitive function was assessed at both time points by the Mini-Mental State Examination (MMSE); cognitive decline was coded as a dichotomous variable and was defined as 1 if the subject's score had declined 3 or more points from the baseline MMSE score at the 1-year follow-up interview and as 0 otherwise. The association between educational attainment and decline in cognitive function over 1 year was examined in logistic regression models that were stratified by age group (< 65 years, > or = 65 years) and by baseline MMSE level (MMSE > 23, MMSE < or = 23). Covariates included age, baseline MMSE score, ethnicity, residence, lifetime diagnosis of abuse of alcohol or other drugs, and gender. In those with baseline MMSE > 23, education was a significant predictor of cognitive decline, not only in the elderly but also in younger subjects. Among those with baseline MMSE < or = 23, education was not a significant predictor of cognitive decline. The fact that education provides protection against cognitive decline even in those younger than 65 years, in whom the prevalence and incidence of dementia are very low, would seem to indicate that education or its correlates provides protection against processes other than dementia that might produce a decline in test performance in young persons.