Breast cancer is the most frequent neoplasm in women. Expression of the estrogen receptor (ER) has a key role in breast cancer; the ER gene is located at chromosome 6q24-q27 and is made up of 8 exons with a total of 140 kb. The polymorphism in codon 325 of exon 4 (ER325) is a transition CCC-->CCG. The objective of this study is to analyze the frequency of this polymorphism in breast cancer using the polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) technology. DNA was extracted from tumor cells of 70 breast cancer patients and from the peripheral blood of 69 individuals without any known pathology (control group). Amplification products of the ER gene were analyzed by SSCP. In breast cancer patients the ER325 polymorphism was detected in 42.8% of the cases. In contrast, in the control group, the frequency of the same polymorphism was 24.6. Statistical comparison of the frequency distributions revealed that they are significantly different (p = 0.023). There was also an association between ER325 polymorphism and the absence of lymph node metastases (p = 0.038). Our data suggest that there is a relationship between the ER325 polymorphism and susceptibility to breast cancer (OR = 2.3; 1.10 < OR < 5.1) and that it can also be related with the metastasization process.