The carcinogenic potential of environmental xenoestrogens (i.e., organochlorines) is a matter of controversy. Their pathogenic role as promoters in breast cancer has been previously suggested. In Europe, despite their prohibition since the '70s, residues persist in soil and rivers resulting in a widespread contamination of the general population. In this study, we have compared the serum levels of p,p'-1,1-dichloro-2, 2-bis (4-chlorophenyl) ethylene (DDE) and hexachlorobenzene (HCB) in 231 women at the time of breast cancer discovery and in 290 age-matched healthy controls. p,p'-DDE was found in 76.2% of cases and in 71.1% of controls but HCB was present only in 12.6% of cases (29 from 231) and in 8.9% of controls (26 from 290). Even if taking all undetectable results (recorded as "0") into consideration, mean values were significantly different in cases when compared to controls. The serum level of p,p'-DDE was 3.46 +/- 3.48 ppb (0.58 +/- 0.58 microg/g lipid) in patients and 1.85 +/- 2.09 ppb (0.31 +/- 0.35 microg/g lipid) in controls (p < 0.0001). The HCB serum level was 0.66 +/-.25 ppb (0.11 +/- 0.21 microg/g lipid) in patients and 0.20 +/- 1.02 ppb (0.03 +/- 0.17 microg/g lipid) in controls (p < 0.0001). When considering p,p'-DDE and HCB as binary variables (1 if higher than the limit of quantification, 0 if lower), the presence of both residues was significantly associated with an increased risk of breast cancer development (OR 2.21, 95% CI 1.41-3.48 for p,p'-DDE and OR 4.99, 95% CI 2.95-8.43 for HCB). No excess was observed among parous women or when familial history of breast cancer was considered. In the cancer group, no differences in serum levels of p,p'-DDE or HCB were found in relation with estrogen-receptor (ER) status, Bloom stage or lymph node metastasis, but the HCB level was moderately correlated with tumor size (p = 0.026).