Background: Cardiovascular disease is a major risk for cognitive impairment. The opioid precursor peptide pro-enkephalin A (PENK-A) is a circulating hormone associated with several cardiovascular diseases and possibly vascular dementia. The objective of this study was to test the association of plasma PENK-A with incident cognitive impairment and describe potential differences by age, race, and sex. Methods: REGARDS is a prospective population-based cohort of 30,239 Black and White adults. Baseline plasma PENK-A was measured in 462 participants who developed cognitive impairment over a median 4.7 years and 556 who did not develop this. PENK-A’s association with incident cognitive impairment was assessed by logistic regression models adjusted for confounders. We tested for differences by age, sex, and race with interaction terms. Results: PENK-A concentration was comparable between cases and controls, but higher with White race, female sex, greater age, coronary artery disease, and chronic kidney disease. Categorical and linear models did not reveal an association with cognitive impairment. Spline plots showed a non-linear association with decreased odds at very high PENK-A values (adjusted OR for 95 th vs 50 th percentiles 0.66, 95% CI 0.46-0.95). This association was only apparent after final adjustment for kidney function. Interaction testing showed substantial interactions by each age and sex (all P < 0.06). In subgroup analyses, shapes of association differed but no independent trends were discernable (Figure). Conclusions: Circulating PENK-A was not particularly associated with cognitive impairment odds. High levels may be protective; this finding could be spurious. Differences by sex and age may exist, expanding upon prior findings in REGARDS of differing non-linear associations with stroke by sex and race. Further work is needed to characterize the importance of endogenous opioid pathways to cardiovascular health and differences by demographic factors.