Background and ObjectivesSerum creatinine is typically used to assess kidney function. Impaired kidney function and thus high serum creatinine increase the risk of poor cognitive performance. However, serum creatinine might have a nonlinear association because low serum creatinine has been linked to cardiovascular risk and impaired cognitive performance. We studied the longitudinal association between serum creatinine and cognitive performance in midlife.MethodsSince 2001, participants from the Cardiovascular Risk in Young Finns Study were followed up for 10 years. Serum creatinine was measured repeatedly in 2001, 2007, and 2011. Sex-specific longitudinal trajectories for serum creatinine among participants without kidney disease were identified with latent class growth mixture modeling. Overall cognitive function and 4 specific domains—working memory, episodic memory and associative learning, reaction time, and information processing—were assessed with a computerized cognitive test.ResultsFour serum creatinine trajectory groups with clinically normal serum creatinine were identified for both men (n = 973) and women (n = 1,204). After 10 years of follow-up, cognitive testing was performed for 2,026 participants 34 to 49 years of age (mean age 41.8 years). In men and women, consistently low serum creatinine was associated with poor childhood school performance, low adulthood education, low adulthood annual income, low physical activity, and smoking. Compared to the men in the low serum creatinine trajectory group, those in the high serum creatinine group had better overall cognitive performance (β = 0.353 SD, 95% CI 0.022–0.684) and working memory (β = 0.351 SD, 95% CI 0.034–0.668), while those in the moderate (β = 0.247 SD, 95% CI 0.026–0.468) or normal (β = 0.244 SD, 95% CI 0.008–0.481) serum creatinine groups had better episodic memory and associative learning. No associations were found for women.DiscussionOur results indicate that in men, compared to low serum creatinine levels, consistently high levels may be associated with better memory and learning function in midlife.