Although mutations in three genes, amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2), have been identified as genetic causes of early-onset Alzheimer s disease (EOAD), there has been a single report on a PSEN1 mutation in Koreans. In the present study, we performed a genetic analysis of six Korean patients with EOAD. Direct sequencing analysis of the APP, PSEN1 and PSEN2 genes revealed two different mutations of the PSEN1 gene (G206S and M233T) and one mutation of the APP gene (V715M) in three patients with age-at-onset of 34, 35, and 42 yr, respectively. In addition, two patients with age-at-onset of 55 and 62 yr, respectively, were homozygous for APOE epsilon 4 allele. One woman had no genetic alterations. These findings suggest that PSEN1 and APP gene mutations may not be uncommon in Korean patients with EOAD and that genetic analysis should be provided to EOAD patients not only for the identification of their genetic causes but also for the appropriate genetic counseling.