Background: Genetic influences on the development of late-onset Alzheimer disease (LOAD) are heterogeneous and ill defined. Objective: To determine the genetic risk factors for LOAD. Design: We asked the following questions: (1) Does early-onset Alzheimer disease (EOAD) occur in families with predominantly LOAD? and (2) Does the apolipoprotein E (APOE) genotype explain the wide differences in onset age in LOAD families? Setting: University of Washington Alzheimer Disease Research Center, Seattle. Participants: A total of 136 kindreds and a separate group of 29 affected parent-child pairs. Main Outcome Measures: We evaluated the kindreds with familial LOAD for the occurrence of EOAD and the affected parent-child pairs with a 20-year or more difference in the age at onset. Results: In the 136 kindreds with LOAD, 104 had only late-onset cases (men, 36%), whereas 32 families (24%) had a combination of LOAD and EOAD cases. The 44 EOAD cases in these families accounted for 20% of cases of AD in the 32 families and 6% in all 136 families. The early-onset cases had a mean ± SD onset age of 56.1 ± 3.2 years (range, 45-59 years; men, 50%). Seven (28%) of 25 individuals with EOAD sampled did not have an APOE ε4 allele, and 2 of the earliest-onset cases were ε3/ε3. In 29 parent-child pairs with a 20-year or more difference in age at onset, 7 (35%) of the 20 children sampled did not have an APOE ε4 allele. Conclusions: Many LOAD families (approximately 25%) have at least 1 individual with EOAD, and in these individuals, the ratio of men to women is nearly 50%, suggesting a possible subtype of familial AD. The APOE genotype plays an important role in these early-onset cases, but at least one fourth of the risk must represent the influence of other genetic and/or environmental factors. These LOAD families with early-onset cases represent an important resource for investigation of these factors. ©2006 American Medical Association. All rights reserved.