OBJECTIVE - Individuals with type 2 diabetes have high risk of late-life cognitive impairment, yet little is known about Strategies to modify risk. Targeting insulin resistance and vascular complications-both associated with cognitive decline-may be a productive approach. We investigated whether dietary [at, which modulates glucose and lipid metabolism, might influence cognitive decline in older adults with diabetes. RESEARCH DESIGN AND METHODS - Beginning in 1995-1999, we evaluated cognitive function in 1,486 Nurses' Health Study participants, aged >= 70 years, with type 2 diabetes second evaluations were conducted 2 years later. Dietary fat intake was assessed regularly beginning in 1980; we considered average intake from 1980 (at midlife) through initial cognitive interview and also after diabetes diagnosis. We used multivariate-adjusted linear regession models to obtain mean differences in cognitive decline across tertiles of fat intake. RESULTS - Higher intakes of saturated and trans fat since midlife, and lower polyunsaturated to saturated fat ratio, were each highly associated with worse cognitive decline it, these women. On a global score averaging all six cognitive tests, mean decline among women in the highest trans fat tertile was 0.15 standard units worse than that among women in the lowest tertile (95% CI -0.24 to -0.06, P = 0.002); this mean difference was comparable with the difference we find in women 7 years apart in age. Results were similar when we analyzed diet after diabetes diagnosis. CONCLUSIONS - These findings suggest that lower intakes of saturated and trans Fat and higher intake of polyunsaturated fat relative to Saturated fat may reduce cognitive decline in individuals with type 2 diabetes.