It is hypothesized that polymorphisms of estrogen receptor-alpha (ERalpha) gene are involved in endometrial cancer. To test this hypothesis, the genotype distributions of six different loci (codon 10 T-->C, codon 87 G-->C, codon 243 C-->T, codon 325 C-->G, codon 594 G-->A, and intron 1 C-->G) of the ERalpha gene were investigated and their association with endometrial cancer was determined. The DNA from 113 cases of human endometrial cancer was analyzed by sequence-specific polymerase chain reaction. The relative risk of variant genotype was calculated by comparison with 200 healthy controls. The frequency of variant genotype on codon 10 was significantly lower in endometrial cancer patients as compared to controls. Nine of 113 endometrial cancer patients (8.0%) showed genotype 10C/C compared to 27 of 200 healthy controls (13.5%). The relative risk of genotype 10C/C was calculated as 0.44, compared to wild-type. Forty-five of 113 endometrial cancer patients (39.8%) showed genotype T/C on codon 10 compared to 111 of 200 healthy controls (55.5%). The relative risk of genotype 10T/C was calculated as 0.67, compared to wild-type. The polymorphism on codon 87 was not detected both in endometrial cancer patients and in healthy control. Other loci, intron 1, and codons 243, 325, and 594, did not show a correlation with endometrial cancer. The frequency of alleles on codon 10 was also significantly lower in endometrial cancer patients as compared to controls. Sixty-three of 226 alleles (27.9%) of endometrial cancer patients showed allele C compared to 165 of 400 (41.2%) of healthy controls. The relative risk of allele 10C was calculated as 0.67, compared to wild-type. Other loci, intron 1, and codons 243, 325, and 594, did not show a difference between cancer patients and controls. All genotype and allelic distributions were in accordance with the Hardy-Weinberg equilibrium. The present study demonstrates for the first time a protective effect of 10C allele against endometrial cancer. Thus, inherited alterations in ERalpha may be associated with changes in estrogen metabolism and thereby may possibly explain inter-individual differences in disease incidences of endometrial cancer.