Objective Few studies have investigated changes in alcohol consumption and risk of cognitive impairment among oldest old adults. Methods In a prospective study of 1309 women ≥65 years old, alcohol use was assessed at repeated visits and used to estimate average change in alcohol consumption over 16 years. Clinically significant cognitive impairment (mild cognitive impairment and dementia) was assessed at year 20. Results Compared with the reference group (slight decrease in alcohol consumption by 0–0.5 drinks/week, 60.4%), increasing consumption over time (>0 drinks/week) was not associated with risk of cognitive impairment (5.0%, odds ratio [OR]: 1.00, 95% confidence interval [CI]: 0.54–1.85). Decreasing consumption by >0.5 drinks/week was associated with increased risk (34.5%, OR: 1.34, 95% CI: 1.05–1.70). Adjustment for age, education, diabetes, smoking, BMI, and physical activity attenuated the magnitude of the effect slightly and resulted in borderline statistical significance. Conclusion Women in their ninth and tenth decade of life who decrease alcohol use may be at risk of cognitive impairment.