Objectives: This brain imaging study examines whether cognitively normal (NL) individuals with 2 parents affected by late-onset Alzheimer disease (LOAD) show evidence of more extensive Alzheimer disease pathology compared with those who have a single parent affected by LOAD.Methods: Fifty-two NL individuals received MRI, 11 C-Pittsburgh compound B (PiB)-PET, and 18 F-fluoro-2-deoxyglucose (FDG)-PET. These included 4 demographically balanced groups (n 5 13/group, aged 32-72 years, 60% female, 30% APOE e4 carriers) of NL individuals with maternal (FHm), paternal (FHp), and maternal and paternal (FHmp) family history of LOAD, and with negative family history (FH2). Statistical parametric mapping, voxel-based morphometry, and z-score mapping were used to compare MRI gray matter volumes (GMVs), partial volumecorrected PiB retention, and FDG metabolism across FH groups and vs FH2. Alzheimer disease (AD) is an age-related neurodegenerative disease and the most common form of dementia. Clinical studies indicate that by the time patients seek diagnosis, the amount of irreversible brain damage that may have already occurred hinders treatment potential. Effective interventions, as they become available, ideally would be implemented in at-risk individuals before symptoms occur.Having a family history (FH) of AD is a major risk factor for developing the disease. 1,2 While the rare early-onset forms of AD (EOAD) have autosomal dominant genetic inheritance, the risk of developing late-onset AD (LOAD) (i.e., .99% of the AD population older than 60 years) is influenced by genetic and nongenetic factors.1 Although LOAD does not show recognizable mendelian inheritance, risk is to some extent genetically determined, as shown by the familial aggregation of many cases.2 Among probands, those with a parent affected by LOAD are at particularly high risk, 3,4 and the risk is higher still when both parents are affected. [5][6][7] However, because only ,5% of individuals have both parents affected by LOAD, 2,5-7 the clinical profile associated with this rare circumstance has been poorly studied and epidemiologic findings have not been characterized by the use of biomarkers.