Mutations in Presenilin 1 (PS1) and Presenilin 2 (PS2) genes account for up to 50% of familial early-onset Alzheimer's disease (EOAD). In order to assess the genetic contribution of the PS genes in a series of Polish patients, we performed a mutational analysis in 6 autosomal dominant (ADEOAD), 8 familial and 41 sporadic EOAD cases from Poznan region. Three missense mutations in the PS1 gene (Ala246Glu in exon 7, Pro267Leu in exon 8, and Leu424Arg in exon 12) were found in patients from families with ADEOAD. In addition, the Glu318Gly noncausative polymorphism in exon 9 was detected in two unrelated sporadic EOAD cases. The variation was also absent from other 53 patients and 48 controls. Therefore, we could not confirm the previous suggestion that the Glu318Gly substitution may be a risk factor for AD.