The Pro12Ala polymorphism of peroxisome proliferator-activated receptor-gamma (PPARG; NCBI dbSNP rs1801282) has been associated with preservation of cognitive function, decreased risk of diabetes, and increased risk of obesity. We attempted to replicate these associations, testing cognitive function and lifetime cognitive change in 519 participants who took the same cognitive test at ages 11 and 79 years. Scores were also available for other cognitive tests at age 79 years, along with history of diabetes, current Body Mass Index (BMI), and other disease and demographic variables. Pro12Ala carrier status was not directly associated with diabetes history or BMI. In carriers who contracted diabetes despite carrying the protective allele, cognitive decline as measured by one test was significantly greater than in other groups. Only six individuals fell into this group; the other cognitive tests did not show this effect. This sample did not replicate the direct association of the PPARG Pro12Ala allele with diabetes status or preserved cognitive function. The data did suggest that risk of cognitive decline is greater when Pro12Ala carriers contract diabetes.