The association of estrogen replacement therapy (ERT) with cognitive functioning was assessed in 6,110 women aged 48-67 years participating in the Atherosclerosis Risk in Communities (ARIC) study, a multicenter longitudinal investigation. ERT was evaluated in relation to results of three cognitive tests (the Delayed Word Recall (DWR) Test, the Digit Symbol Subtest of the Wechsler Adult intelligence Scale-Revised (DSS/WAIS-R), and the Word Fluency (WF) Test) using data from the first follow-up visit of the cohort (1990-1992). No consistent associations were seen between ERT and either the DWR test or the DSS/WAIS-R after adjusting for age, education, and additional covariates previously found to be associated with cognitive function scores. Among surgically menopausal women aged 48-57 years, adjusted mean WF scores were slightly greater in ERT current users (mean WF 35.9) than in never users (mean WF 33.5) (p < 0.02); and within current users, adjusted WF scores increased with duration of ERT use. However, the finding that ERT was associated with a slightly higher level of performance on only one of three measures offers little support for the hypothesis that ERT has a major protective effect on cognitive function in women less than 68 years of age. The generalizability of these findings to older women who are more likely to experience cognitive decline and who may be using ERT for longer periods of time is limited by the relatively young age of the cohort.