The presence of estrogen receptor (ER) is a well-known predictor of clinical outcome in human breast cancer. We examined the ER gene in 26 primary breast cancers (14 ER-positive, 12 ER-negative) to determine if alterations of the gene are associated with the ER-negative status. In tumor biopsies and peripheral blood DNA obtained from the same patients we analyzed the ER exon structure using polymerase chain reaction amplification, restriction endonuclease digestion, and agarose gel electrophoresis. All blood and tumor samples, regardless of ER status, showed a complete set of eight exons of normal sizes, ruling out deletions or rearrangements of the ER gene in excess of ±20 nucleotides. Previous reports indicate that the two-allele ER PvuII polymorphism could be associated with ER expression in breast cancer (Hill et al., Cancer Res., 49: 145–148, 1989) as well as with patient age at time of tumor diagnosis (Parl et al., Breast Cancer Res. Treat., 14: 57–64, 1989). We localized the PvuII polymorphism in intron 1, 0.4 kilobase upstream of exon 2. Sequence analysis showed the polymorphism to result from a point mutation (T→C) at the fifth position of the restriction site (CATCTG). We determined the PvuII restriction fragment-length polymorphism genotype in 257 primary breast cancers and 140 peripheral blood DNA samples obtained from women without breast cancer. The results indicate that the PvuII polymorphism is not associated with ER content or patient age at tumor diagnosis.