In early‐onset familial Alzheimer's disease pathogenic mutations have been found in the amyloid precur sor protein (APP) gene and in the presenilin (PS)‐1 and PS‐2 genes. We screened for mutations in these genes in 20 patients with familial AD from the Finnish population. In addition, we sampled 41 sporadic AD patients and 59 controls to test for mutations identified in our familial AD cases. We detected an A‐to‐G transition in the PS‐1 gene, resulting in a glutamic acid (G1u)‐to‐ glycine (Gly) substitution at codon 318 in 2 familial and 2 sporadic AD patients. The Glu318Gly mutation has previously been reported to cause AD. We also found the Glu318Gly mutation in 4 healthy aged controls (range, 74–87 years). We thus conclude that the mutation is most likely a rare polymorphism not related to AD.