Estrogens may be implicated in the development of Alzheimer's disease (AD). Most of their effects are mediated via receptors whose function and expression may be modified by DNA polymorphisms. Here the estrogen receptor 1 gene (ESR1) polymorphisms XbaI and PvuII were analyzed in 214 AD patients and 290 controls. In logistic regression analysis, a significantly increased risk of familial AD due to interaction between the ESR1 xx genotype and the apolipoprotein E epsilon4 allele was observed in women in a Swedish clinic-based sample, taking subjects who had neither the xx genotype nor epsilon4 as reference (OR 11.3, 95% CI 2.9-43.8). The risk of AD was more pronounced in early-onset (OR 22.0, 95% CI 3. 7-132.7) than in late-onset (OR 6.0, 95% CI 1.2-29.7) female patients. For women carrying the pp genotype together with epsilon4 the risk of AD was similarly elevated. Likewise in a Swedish community-based set of women, an increased risk of familial AD was observed in subjects who had either the ESR1 xx or pp genotype together with epsilon4. Furthermore, the Pp genotype frequency was found to be significantly increased in Finnish women with sporadic AD. We, thus, conclude that the ESR1 gene may have a role in the development of AD in females.