SummaryCentral nervous system IgG synthesis rates were determined for 51 patients with autopsy-confirmed Alzheimer disease and 23 age-matched controls. Rates were no different between patients and controls when overall groups were compared. Age-at-dementia-onset data were available on 37 patients. Comparisons of 11 early-onset (< 65 years of age) patients with 26 late-onset patients revealed significantly increased intrathecal IgG synthesis rates for the late-onset group. These results suggest that IgG synthesis may be a contributing pathogenic factor in a subgroup of patients with late-onset Alzheimer disease.