© 2017 The Authors. Circulation is published on behalf of the American Heart Association, Inc., by Wolters Kluwer Health, Inc. BACKGROUND: Despite concerns about adverse neurocognitive events raised by prior trials, pharmacological PCSK9 (proprotein convertase subtilisin/kexin type-9) inhibition was not associated with neurocognitive effects in a recent phase 3 randomized trial. PCSK9 loss-of-function (LOF) variants that result in lifelong exposure to lower levels of low-density lipoprotein cholesterol can provide information on the potential long-term effects of lower low-density lipoprotein cholesterol on neurocognitive impairment and decline. METHODS: We investigated the association between PCSK9 LOF variants and neurocognitive impairment and decline among black REGARDS study (Reasons for Geographic and Racial Differences in Stroke) participants with (n=241) and without (n=10 454) C697X or Y142X LOF variants. Neurocognitive tests included the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) battery (Word List Learning, World List Delayed Recall, Semantic Animal Fluency) and Six-Item Screener (SIS) assessments, administered longitudinally during followup. Neurocognitive impairment was defined as a score =1.5 SD below age, sex, and education-based stratum-specific means on 2 or 3 CERAD assessments or, separately, a score <5 on any SIS assessment at baseline or during follow-up. Neurocognitive decline was assessed using standardized continuous scores on individual neurocognitive tests. RESULTS: The mean sample age was 64 years (SD, 9), 62% were women, and the prevalence of neurocognitive impairment at any assessment was 6.3% by CERAD and 15.4% by SIS definitions. Adjusted odds ratios for neurocognitive impairment for participants with versus without PCSK9 LOF variants were 1.11 (95% confidence interval [CI], 0.58?2.13) using the CERAD battery and 0.89 (95% CI, 0.61?1.30) using the SIS assessment. Standardized average differences in individual neurocognitive assessment scores over the 5.6-year (range, 0.1?9.1) study period ranged between 0.07 (95% CI, ?0.06 to 0.20) and ?0.07 (95% CI, ?0.18 to 0.05) among participants with versus without PCSK9 LOF variants. Patterns of neurocognitive decline were similar between participants with and without PCSK9 LOF variants (all P>0.10). Odds ratios for neurocognitive impairment per 20 mg/dL low-density lipoprotein cholesterol decrements were 1.02 (95% CI, 0.96?1.08) and 0.99 (95% CI, 0.95?1.02) for the CERAD and SIS definitions of impairment, respectively. CONCLUSIONS: These results suggest that lifelong exposure to low PCSK9 levels and cumulative exposure to lower levels of low-density lipoprotein cholesterol are not associated with neurocognitive effects in blacks.