Patients with Alzheimer's disease (AD) have elevations of fasting plasma insulin that are hypothesized to be associated with disrupted brain insulin metabolism. We examined paired fasted plasma and CSF insulin levels in 25 patients with AD and 14 healthy age-matched adults and determined whether insulin levels were related to severity of dementia and apolipoprotein E-ϵ4 homozygosity, a known genetic risk factor for AD. The AD patients had lower CSF insulin, higher plasma insulin, and a reduced CSF-to-plasma insulin ratio when compared with healthy adults. The differences were greater for patients with more advanced AD. Patients who were not apolipoprotein E-ϵ4 homozygotes had higher plasma insulin levels and reduced CSF-to-plasma ratios, whereas ϵ4 homozygotes with AD had normal values. Both plasma and CSF insulin levels are abnormal in AD, and there are metabolic differences among apolipoprotein E genotypes.