Genetic Differences (GenDiff) was an epidemiologic case control study. Cases (235) were newly recognized (e.g., “incident”) “Probable AD” (McKhann criteria NINCDS-ADRDA). Subjects were discovered and diagnosed by the Alzheimer’s Disease Patient Registry from a community based HMO. Controls were selected at random from the same HMO, without cognitive impairment, and frequency matched for age and sex. The cohort was derived from the same population as the Adult Changes in Thought (ACT) study. They were also cognitively screened and followed. Blood samples were obtained on most that entered GenDiff with consent for any future genetic analyses.