Previous studies suggest that cholesterol metabolic dysregulation, characterized by abnormally low or high serum total cholesterol (TC) values, constitutes a risk for pronounced cognitive decline in old age. We tested this prediction using a population-based representative Swedish sample (N = 382), born in 1901-1902, and subsequently assessed on TC and 3 cognitive outcomes (verbal ability, spatial ability, and perceptual-motor-speed) at ages 70, 75, 79, 85, 88, and 90. None of the participants were on lipid-lowering medication, as prescription availability for these medications were not initiated in Sweden until the mid-1990s. We used a 3-level hierarchical model, with cognitive tests nested within time, nested within individuals. Estimates from this model revealed a nonlinear between-person association between TC and cognition, indicating that low, and to some degree high, TC values were associated with poorer cognition. This association was stronger among nondementia-cases (n = 255). Among subsequent dementia cases (n = 127), the data suggested a linear trend, indicating that lower TC values were associated with poorer cognition. TC levels declined over time in the vast majority (96%), and the steepness of this decline was associated with the rate of cognitive decline. This within-person association was particularly strong among incident dementia cases with low TC values. Our findings indicate an optimal range of TC values associated with better cognition in old age and that the within-person association between TC and cognition is related to dementia pathologies. Further, our findings demonstrate the need to separate between-person from within-person associations when evaluating the relation between TC and cognition in old age. (PsycInfo Database Record (c) 2020 APA, all rights reserved).