An Alu insertion polymorphism of the progesterone receptor (PR) wasreported recently to be associated with a reduced risk of breast cancer, with risks of 0.8- and 0.3-fold associated with the heterozygote and homozygote genotypes, respectively. This intronic variant is considered to be in linkage disequilibrium with an exon 4 hinge region G to T Val660Leu polymorphism. We investigated whether the exon 4 PR polymorphism was associated with breast cancer in Australian women, using a population-based study of 1452 cases and 793 controls, half of whom were <40 years of age, and the other half were 40-59 years of age. There was no difference in genotype distribution between cases and controls (P = 0.5) and no evidence of risk associated with either the GT or TT genotypes compared with the common GG genotype. The adjusted odds ratios (ORs) were 0.97 (95% confidence interval, 0.79-1.19) and 1.52 (95% confidence interval, 0.87-2.66), respectively (P = 0.8 and 0.1), and the results were independent of age and family history of breast cancer. Our data provided no support for the previously reported decreased risk of breast cancer associated with the T allele, with 80% power to detect an OR of 0.8 or less for the heterozygote genotype and 90% power to detect an OR of 0.3 or less for the rare homozygous TT genotype. There was also no support for a greatly increased risk of breast cancer associated with the T allele, given that we had 80% power to detect risks of 1.3 and 2.0 associated with the GT and TT genotypes, respectively. We therefore conclude that this polymorphism is not associated with a markedly reduced or increased risk of breast cancer in Australian women <60 years of age. However, despite its considerable size, our study cannot exclude a small reduced or increased risk associated with the T allele, especially the rare TT genotype.