Beta amyloid (Abeta) protein accumulates in the brains of individuals with Alzheimer disease (AD) and is detectable in cerebrospinal fluid and plasma. To examine plasma levels of Abeta peptides Abeta(40) and Abeta(42) as predictors of incident AD and other types of dementia. Prospective, population-based cohort study. The Uppsala Longitudinal Study of Adult Men. Plasma Abeta(40) and Abeta(42) levels were analyzed as predictors of incident AD in 1045 men at age 70 years and 680 men at age 77 years using Cox proportional hazards analyses. Alzheimer disease and other types of dementia were diagnosed by standardized screening, clinical evaluation, and medical record review. Hazard ratios of AD (primary outcome) and vascular dementia or other dementia (secondary outcomes) according to baseline levels of plasma Abeta(40) and Abeta(42). From the age of 77 years at baseline, 46 individuals developed AD at follow-up (median, 5.3 years). A low plasma Abeta(40) level at age 77 years was associated with higher incidence of AD. The multivariate-adjusted hazard ratio was 4.87 (95% confidence interval, 1.63-14.6) for the lowest Abeta(40) tertile compared with the highest tertile. On follow-up from age 70 years at baseline (median, 11.2 years), 82 individuals developed AD. Plasma Abeta(40) and Abeta(42) levels measured at age 70 years were not significantly associated with incident AD. Low plasma Abeta(40) levels predicted incident AD in elderly men independently of potential confounders. Plasma Abeta(42) levels were not significantly associated with AD incidence. The clinical value of Abeta measurement in plasma remains to be established in future studies.